Love Twitter, hate Facebook with a passion.
Facebook wants to know everything about you. Friends, life history, job, even freaking religious views and hobbies. And now they want to make it all public to the world.
Twitter doesn't care about any of that. You sign up, you follow interesting people and you talk about cool stuff. Its fantastic. What ever subject interests you, you'll find a group of interesting people talking about it.
For the record, I use the free version of Twitterific on the iPhone.