It's not for everybody to tweet, it's for everybody to follow. The more people figure that out, they see it's RSS-plus. It's literally the place you check for information.
Facebook has stitched together your social graph. The idea of an interest graph is to bring together everyone that has shared interests. If I can isolate the people who are into mountain biking in Marin, in one place, the ability to put ads against that is really high.