[grsites]

This is not terribly surprising, and illustrates why Facebook and Twitter are completely different (and complementary) social networks.

Facebook for keeping tabs on people you know and who know you, i.e. symmetrical relationships.

Twitter turned out to be well suited for keeping track of people you know, but who have no idea who you are, because they are well-known, not you, i.e. asymetrical relationships.

[markkat]

This was my thought as well. And thus, I think it's hard to see how they are in competition with each other, aside from competing for users' attention in general.