Mashable asks and answers the question, which one do you use, and when?
I agree with most of the post, I use both, and love both. Facebook is more about people I already know, staying in touch, and having fun. Twitter is fun too, for sure, but I use it more to get information, learn, and connect with people I don’t know who are either in my city or interested in the same things as I am.
I find both have changed a lot about how we communicate, how we stay in touch, and how we use the web.