Yes. I think they befriended me because I worked together with them on a project, or have had interaction in the chat. One was totally random, but I figured I'd befriend them just to see what would happen. I figured there's little harm in doing so at this stage.
I prefer to keep my friends list down to people I chat to regularly, or have had memorable chats with in the past... sorry if newbies try to friend me & I ignore. :3
I originally limited my friends to people I was 'aware' of outside of Glitch -- real friends; flickr, facebook and twitter 'friends' etc. I've since expanded that to include some of the Glitch team and some others - in part due to the achievements that require you to have lots of friends.
Equally, I've tended not to accept random friend requests, and I have a few sitting in my Requests folder...
Mostly the friends I've made have been friends that I added first...usually because I talked with them a bit in chat, they helped me in-game, or I just liked the cut of their jib. Gib? Jib.
I thought people friended others to get badges? There are a few for number of buddies made. I accept friend requests now, since I don't know any of you outside of the game, but I don't really do anything beyond that. Out of alpha, I'll probably be a lot more selective.