I had chatted with someone who is no longer listed as a member of Agnostic.com - also had talked on phone many times and messaged on Facebook - suddenly is no longer on my FB friends list and did not respond to the voice mail I left. I have a feeling that mental and physical health issues had some kind of impact. I feel a bit concerned a bit sad, but that kind of stuff happens.
If you think you are justifiably worried maybe contact someone else this person is close to?
We had not met in person and live hundreds of miles from each other - we had talked on the phone maybe a couple dozen times and I really enjoyed the phone chats. I left a message on voice mail, so I did what I could .... it is what it is.