Senh: I've been preaching this since ... forever: "Click-thru rates for ads on social networks have always been really low, so they can’t charge nearly as much as Google per pageview. That hasn’t changed since social networks entered the internet landscape and won’t change in the future."
So far, I’ve only heard of two people who are willing to pay for Twitter - Guy kawasaki and Roger Ebert. If you search on Google, you’ll probably find more, but those are the only two I’ve read about. They have one thing in common - they each have over 100,000 followers.