I think Facebook will REALLY take off if they can connect to users of Gree.jp or Mixi in Japan, the two most popular social networking sites in that country.
Again that depends on what people are doing with it.
When I first started on Facebook I played some of those stupid apps like that "hatch and grow a pet bird" thing. I know lots of people who are on FarmVille (and all of the spinoffs like CityVille). I now block all invitations from apps like that because if I don't, I get spammed with requests. THIS is the sort of thing that I think (I hope?!) people will tire of and realize is a passing fad.
What Facebook is good at is keeping your social circle a bit bigger than it normally would be. I don't need Facebook to know what my immediate family and closest friends are doing because, well, I talk with them all the time. But my friends that I otherwise don't see or talk to very often, or the people who used to be really good friends in grade school or university -- were it not for this casual connection on Facebook, there would be no connection at all.
THAT is why I use it, and that is why I will keep using it... but if that basic concept starts falling into the shadows of whatever else they come up with, then it will be time to leave.