I think the idea of value added services makes more sense than advertising, but I'm still skeptical about this idea for Twitter:
Stone said on Monday that Twitter would remain free for consumers and businesses, and that the company's main focus at the moment is developing new features for commercial users, such as "lightweight analytics" and a directory of commercial accounts that would verify that businesses on Twitter are legitimate.
The problem is that there's a precedent out there: Google Analytics, which are pretty good, and free. For Twitter to be able to charge, they would really have to add value. I guess we'll have to wait and see.