My mom tagged me in an advertisement because she wants grandkids
I think there was a link on Reddit that I clicked.
By searching for and liking atheist groups on Facebook:
Atheist Edge; and
Atheist Community Organization.
Then Agnostic.com ads were posted in my Facebook news feed.
Actually, it turns out I lied. It's what came to my mind when I typed my response, but I had been reading other people's responses and this stuck in my mind. In reality, I'm pretty sure it was somewhere else like Reddit. But it's long enough ago we will likely never know for sure.
Advert on Facebook, and a mention from an online friend (who doesn't seem to actually be a member... but that's okay, since I reached the promised land and he can only look from afar.)