I like Facebook because it's easy to talk to people about things and you get an interactive feel to it all. If you want to play games, listen to music, look at humorous pictures, join groups and talk about sports, or politics, or your hobbies, or just keep up with high school, college or internet friends, it's easy.
Twitter is good for humorous one-liners, up-to-the-second news, and new topics and trends.
I prefer FB to Twitter because it's more visual and easier to communicate with people.