IMO, Computer world made an interesting point yesterday in their “The_Business_Sense_of_Twitter” article:
"It's not cool on Facebook or LinkedIn to friend people you don't know at all. But Twitter is about information, and "following" people you don't know -- merely because of the interesting things they might say -- is the rule."
FB and LI are oriented on the relation between people: you need to know someone a little to add it to your network and “listen” to him or her. Twitter is centered on hearing the message first. Definitely complementary tools.
Pointing the obvious? Well, so much noise made comparing the Twitter and Facebook, I think it does not hurt.