Personally I think celebrity endorsement style advertising is a good match for Twitter. They have all these celebrities with all these fans. On the way to work I think I heard the DJ hawk 2 or 3 different products. I think that's about the only model that doesn't feel forced. Find out what the people with 20million followers like and find out how much those companies are willing to pay for them to talk up the product. Give some endorsement kickback to the celebrity.