I also game with a lot of people online whose real names I don't know, or whose last names I don't know. My tabletop games are with family (my wife and kids) and people I met at a gaming store whom I now consider friends -- we've been meeting for the past three years.
at conventions ... i've gamed with hundreds of complete strangers.
the entire group i game with now i met online in a way. they are friends now. but when they first came over to game, i didn't know anything about them. some not even their name until they introduced themselves at my door.
I voted friends who became so through gaming - but it hasn't always been that way.
My original gaming group was friends first, gamers second. But, after we moved out here to the DC metro area, gaming became the way that we made friends - it was a good social activity with some loosely enforced norms that let us meet people our own age that had similar interests.
2 friends whom I met at church (one of whom is now a coworker), my wife, one guy I went to high school with (and subsequently lost touch with for 10 years), his wife, a guy who answered a call for players, and his wife. All of whom are friends now.
Home game where I DM: my wife and 5 current or formwer co-workers, all of who are friends outside of gaming and work.
Game I play in #1: 1 former co-worker and 5 others I game with, and only know from gaming.
Game I play in #2: complete strangers, except for 1 guy who I knew from around campus, then bumped into at GenCon. He freaked out when I bumped into him. Don't know if was more shocked that I gamed or that he was "outed".