The big difference is that Facebook lacks a native SMS implementation, and also lacks a lot of third party apps to interact with the service. I never post to Twitter via their site, it's always via a program either on my Mac or on the iPhone. Facebook is also a mostly closed loop type of setup, whereas Twitter has some value in the openness of most users posts.

Overall, I've found value in both Facebook and Twitter. Sure, I could go make my own site somewhere and post similar things, but the centralization the two services offer is a big deal.