Why aren't facebook users obsessed with Twitter yet?

Every time someone asks me about Twitter I wonder how come more people aren't interested in using it. Facebook has obviously blown up - everyone of every age is on it, and using it often, whether they claim to like it or not. People love to write on each others walls, update their statuses, and comment to each other about both. So in that case, why don't people start using twitter? Aside from the fact that through twitter you can update your Facebook status, it's essentially a combined Facebook wall and Facebook status, but through texting via a cell phone, which almost everyone I know is also obsessed with. It takes many of the things that people do on Facebook and makes them faster- your friend updates his status, and you know immediately. And if that status is "out at a particular bar" then you might find you're in the area and make your way over when it matters, not when it's too late three hours later. Your friend writes on your wall and you know immediately, and you can respond just as fast. So if Twitter in it's most basic form combines many of the things that people I know love, how come no one is using twitter?

As I'm writing this, I'm thinking - maybe it's because all we really care about on Facebook are the pictures.