They serve different community niches, though. You seem to think that each must necessarily overlap the uses of the other so significantly that one of them has to fail for the other to succeed.

The functionality of each is so distinct from the other, for the most part, that they're more likely to develop means of benefiting from the existence of the other than to have any chance of crushing the other. In fact, since Facebook already has rudimentary Twitter-like capabilities, I'd say Facebook is more likely to be a Twitter killer than the other way around, because the main attractions of Facebook are not the Twitter-like capabilities.

. . . not that I'm particularly interested in either. For those who are, though, I can see how each has its own, separate, not necessarily competitive attraction.