See, when you do things on Facebook, it’s on Facebook Majority of the time. Most people access twitter from Mobile and Desktop API’s which cause them to spend Less time on the actual website.
Also, They upload pictures to Twitpic/Tweet Photo which is Not Twitter and has little to do with Twitter. I’ve seen people complain that Twitpic wouldn’t allow them to upload. The Twitpic people put out a statement saying “That’s a Twitter Problem, Not A Twitpic Issue” .. <== Basically! ..lol
The reason I did Join twitter is because it’s a good tool for Loging in to sites allow open ID sessions. I do the same with Facebook. . Majority of my Tweets are actually Youtube Comments. lol