Same thing, really. There are two times to kill a company:
1.) When it's small and not established yet.
2.) When it's big and complacent.
FaceBook is neither at the moment, so it's not really a good target to compete against. But that doesn't rule out them becoming #2 in the future. (Particularly since Zuckerberg is quite a bit more arrogant than, say, the Google founders.)
I'm not a huge believer in ____-killers, at least not via direct competition. If you're small and not established, no one even mentions you in that blank, so that's out. In the case where you're big enough that people could consider you worthwhile attempting to kill, you're well-enough entrenched that no one will beat you at your own game. The way you have to die, instead, is by some alternative that wittles away at your core business. No one will kill Windows by making a better desktop OS, and no one will kill Facebook by making a nicer student networking site.
1.) When it's small and not established yet.
2.) When it's big and complacent.
FaceBook is neither at the moment, so it's not really a good target to compete against. But that doesn't rule out them becoming #2 in the future. (Particularly since Zuckerberg is quite a bit more arrogant than, say, the Google founders.)