Size doesn't matter. Utility does. If Facebook were to start completely over again against the current competition, I think they would still rise to prominence without much trouble.

What makes Facebook different, in short, is that while other services offer a place to talk, Facebook is built as a utility in a way other sites aren't. It's the new big communication medium. The people that use it are using it as a replacement to email and IM. That's a huge deal, because it suggests Facebook has the potential to last as long as email has.

Your suggestion that people abandon things all the time is countered by AIM, which has been the prominent IM service for a decade.

Size matters if you need it to become an enabler. You can't get sick of email and leave, for example. Backpackers are constantly using facebook to keep track of their rapidly evolving social sphere. You can rely on most of those you meet being on facebook. This lets you be confidant that you will be able to use facebook to meet up at a later time or organise to do something at some later point. It doesn't matter if you like writing on people's wall in this case. It's hard to backpack without it. That's a consequence of size.

If facebook is how office drinks get organised, you need ot be in on that. (size doesn't matter so much for that though)