It seems to be flavour of the month, but there is definitely something about Facebook which appeals to me in a way that Orkut, LinkedIn and MySpace don't.

I think that what I like about it is that in many ways it feels like its more about your friends than it is about you. To a certain extent all of these social sites feel like a big exercise in self-publicity: "look at me, I do this, I like that, blah blah blah". I'm sure that's true of Facebook too, but for me at least it does seem to genuinely encourage interaction in a way that the other sites don't.