I've met people the old fashion way who ended up not being who I thought they were.

I think your ability to know someone online is very limited.
ETA: You can only know what someone wants you to know, true or false, real or imagined.

From my experiences, I have concluded that you don't really know someone until you meet them face-to-face. There may be exceptions, to a point, but I agree with @Davesnotthere that you get to know the online persona that they present. I prefer the old fashioned way of meeting people, but like the idea that you can meet people you probably never would have met by putting yourself out there online. You just have to be more careful. People can be whomever they choose to be online. Sometimes it is very close to the truth, and sometimes it is nowhere near the truth.

When creating an online persona, people have the benefit of putting only their best self forward in a way that would not be possible in face to face interaction. When chatting, you have time to think through each response and their is no body language available to gather information. Therefor, I don't believe it's possible to get to know a person to their core with online interaction only. Even if the person is being as genuine as possible, there is simply less information at hand to draw from when making a judgement about a person.

Can you ever really know ANYbody that much, no matter how people meet ? So often we hear of someone totally taken by surprise by someone they've been with for many years.

My last serious relationship was with someone I met online. However after a couple weeks, we agreed to continue on the phone - so much more can be learned that way ! We met in person a couple weeks after that. "click".
As it turned out, we were together 4 1/2 yrs.
And yes - there were some surprises toward the end ...

Not that well and you certainly cannot gauge chemistry without meeting. I meet most women I date online. I won't meet anyone in person until I talk to them on the phone.

Often, people use their best pictures online, sometimes from years ago. To the point of not being recognizable

Online, someone may be helping them write. It is much easier to tell by talking if they are as articulate as they appear to be. If they were long distance, definitely video chat before agreeing to meet.

And if they come to you, until you see their place and how they live, you don't have an idea of how they would be if you were together. I dated a woman and when I saw her place, I was done.

I use OkCupid. They have a match % that is pretty accurate if you've both answered a bunch of questions. 80%+ you will probably have a tolerable meeting, 90%+ you will enjoy each other's company but there may or may not be chemistry. The woman I am dating now was a 98% match.

Good luck!

I have not met one online that I feel close to and don't think I will. However, my late partner and I were introduced through a phone call. It was a long distance relationship so we only talked on the phone for 2 months before actually meeting. There was a goal so it was important for each of us to be totally honest. A relationship based on lies is not a relationship.

The funny thing was that when we did finally meet things did not go well. We were living in different cities (Seattle and Dallas) and the cultures were different so our way of looking at things was different. The biggest problem, and I think this can be true online, is all of a sudden the romantic ideals are over and one has to look at making a real life change. That can be daunting. But one often has to step outside out of one's comfort zone to go forward.

I think the longer you spend "getting to know someone" online, the more you form and reiforce your involuntary imaginings of the person, which evetually form unconscious expectations. They way they walk, smell, smile (or not) at waitstaff, etc... So many things you couldn't possibly know by talking about it or aren't likely to discuss. No one can actively present themselves to you exactly the same way they'd appear to your passive observation.

If you want to have a shot at something "real" with someone, I think it's in your best interest to meet them face-to-face ASAP. If you've gotten to know someone so well, just from online contact, that you've decided you want a meatspace relationship, it may already be too late: you may have fallen for your idea of that person (a.k.a. someone who probably doesn'texist) or you just fall in love way too easily and that's a whole 'nother kettle of fish...

Full disclosure: I have tried online dating one time. I met the guy after we exchanged a few messages and we ended up dating about two months. My aforementioned attitude about online relating is why I never put much effort into it, nor bothered to speak to anyone >50 mi away.

I developed the class and taught interpersonal communication online for my college when the consensus was "Duh! No one can be 'interpersonal' using a computer. It has to be in person." I'm not saying there aren't short-comings, difficulties, and a whole lotta work. But it can be done online. So yes, we can get to know somebody online. Jus' sayin'.

You don't. You get to know the online personna they present.

I don't think anyone here will be surprised when I say that humans are complicated. We're the most dangerous primates on the planet and we have a god complex along with huge insecurities that we brought with us when we got chased onto the plains by the apes who were actually good at being apes. It makes us pretty mental at the best of times. So, that being said, you can learn and learn and learn about someone and they will still have the ability to surprise you. You think someone is the bees knees (is that still a thing?) and then you move in with them and you learn all kinds of things that you didn't know, or you learn that the stuff you knew was actually harder to live with than you believed. Getting to know someone is constant work, and you'll never know how well you're going to mesh until you live with someone. The happiest married couple I knew each had their own homes... They loved each other but not so much how they ordered their home, so they did the unthinkable and got separate places while still married. It worked for them; your results may vary.

The point is, yes, online is a good place to meet people. A lot of us are really shy and wouldn't dream of talking to someone that we might consider out of our league, but online, the pressure is lower. So we talk. And we learn. And hopefully, we all tell the truth as best we can. And then we take the next step and the process starts anew. And again. And again... until you're decades (hopefully) down the road, with years of love and joy behind you, and one of you is saying goodbye for the last time as the other vanishes into the dark. It's all we can hope for. It isn't easy. It's often painful and usually fails, but we have to keep trying, I think. What else is this beautiful world for but to share it with someone you love?

Don't know if I really answered your question, but this is what your question inspired in me, so there you have it.

It depends. If both participants in an online friendship are honest and open, I can see no reason they couldn't get to know one another as well as through any other medium or in the physical world.

However, without meeting "IRL," neither of you can be any more than 50% sure the other one isn't who they say are, either because most of us probably subconsciously portray ourselves how we see ourselves rather than how others see us (and also, as @Stinkeye_A points out, because you'll "form and reinforce your involuntary imaginings of the person" ) or because of intentional lying.

As an example, I doubt any of you have guessed that I'm actually an 81-year-old woman living in Buenos Aires.

I think most people aren't really honest with themselves. Otherwise, we'd have a lot less theists so I don't think you can get to know anyone very well through online communication.

I met my first truly serious girlfriend online and we met for real after 2 years of using text, phone and Skype. We talked a lot in those first 2 years and so when I went to see her there were slight physical differences because the webcams back then were rubbish but I was utterly bowled over by how much more beautiful she was in person. Also, there were no real surprises about her or what I knew about her - her apartment was bigger than through the webcam but we got along just great and were jumping into bed after the first 4 or 5 days. We kept the relationship going a long time (10+ years) and are still friends. As time went by certain behaviours and tensions caused me to call an end to it but it wasn't anything to do with the online nature of it, though clearly it lends a different dynamic to the relationship and you have to be very forgiving and tolerant on both sides to make it work.

I like sites like this: showing the inside of my head. Real-life sometimes does not allow honesty.

Some people are much more honest online and some are exactly the reverse. You can't know for sure unless you meet in rl

I've found that following people I know on Facebook is a surprise. Generally a surprise I'd rather not know, so I follow very few people I know. It's just easier that way.

Not well enough

Ok met someone online here and getting to know him as we speak. Did it once before and after a year I asked him to move out. If someone lies about who they are and what they do for a living you've got a non-starter. Lying, deception and broken promises don't make the grade. Have learned not to jump into a relationship. Two months of messaging and phone calls to figure out if I wanted to meet someone. Some of my friends think I'm still the adventurous one from long ago. The key @ElizabethI is being honest from the beginning-no games-be the real you and expect the same. You deserve it.

They say it takes about a year before someone feels comfortable enough to let their guard down and show their true self... So, with that in mind. You can say whatever you want behind a computer monitor and keyboard, but until I have had ample enough time to get to know you one-on-one in person... Because actions speak louder than words.

I have had mixed results. My last serious relationship was 8 years and we met on an online dating site. It moved to texting and then phone calls. We met within a week and were serious quickly. He soon deployed to Iraq and we wrote letters. I enjoyed the letter writing and felt we shared well in the writings. As many have already stated, people will always surprise you in some ways because hopefully, people are growing and changing as they age and go through life. That is after all the point.
That relationship ended due to a relocation and not being willing to leave a child and move too. I find myself online dating again, and this time has been very different. I guess the new norm is to misrepresent yourself online. I have decided that more time is needed texting and talking before meeting in person. Give myself more time to see if what they say stays consistent because the guys I have decided to meet have been a bit off from how they present themselves online and in texts. (read that as Nucking Futs!) LOL.

Discussion is discussion, phone calls, emails, online chat, that is how you get to know people, you can send photos, do video chat. I think you get to know people real well and get real close. I find the longer I have known them online, the fewer the surprises.

Guessing it depends how long you've 'known' them on line. Must be difficult to keep a catfish type thing going over any period of time. And, as others have pointed out, you can 'know' people in real life only for them to go and do something that you went expecting

Not very well. It’s easy to edit yourself.

Some people are much more honest online and some are exactly the reverse. You can't know for sure unless you meet in rl