what's a friend?

I've been browsing facebook recently - adding people to my list of friends. This poses a question - how well do you know someone before calling them a friend? Ben McKay has 215 friends as I type this. I have 62. My barrier has just been substantially reduced. I'll now facebook-friend anyone who has a name and face I recognise. So if I just know your name that's not enough, the same with just face. But if I know both - and preferably knew the match - then that's friendship as far as facebook is concerned.

That's really, really not what real friendship is. I'm still not sure what friendship proper is. At what point does someone go from acquaintance to friend? I used to think it was when you'd been to their house... but I have friends whose houses I've never been to. It's quite curious, as a whole.

Thoughts?