Facebook is games, and photos, and chat, and events, and groups, and coca cola's official page. There are a lot of reasons to visit facebook.
Twitter is short status messages. Basically twitter is about half of what facebook was 5 years ago.
I think the 200 Million number is dishonest. I'm a twitter user, and have been for at least a couple of years...but I think I log into the site and look at it about once a week (here is my twitter: http://twitter.com/blhack). Most of those logins happen over my phone, meaning I don't see the "trending" ads...or any ads at all.
I said that I don't really use twitter. This wasn't always the case. I used to check it probably 10 times a day...but then a new service came along and replaced it for me. It does a better job at status updates...so I switched (and so did most of my friends). This is also a huge problem for twitter. What is twitter giving me that I can't get elsewhere (in my opinion: nothing...doubly so because the new layout is so hideous)