Obviously BOTH! Don't just stop there, look into social bookmarking as well. Have a read over here - Social Media Marketing.
Each can have great or little value to anyone. Twitter for stuff within 140 characters and Facebook for stuff 141 characters and longer. My preference is Twitter.
Both are good and popular, but I prefer using facebook over twitter. I like facebook features and it's more easy to use.
Facebook is so much better, hands down, in terms of features. FB is more than a social networking site. You can also do other things in it like playing games, quizzes, chatting, etc.
I believe you are asking about twitter? as per my opinion, Facebook is much better because twitter is converting into a SPAM machine day by day I am not saying that facebook is SPAM free, but yes its far better when compared with twitter for the same
I would say Twitter as Google is crawling this site all day every day, you've got to remember that the majority of Facebook profiles are private and in the SEO news last week it was said Facebook isn't a factor. (Go to SEO Roundtable for the source).