The latest ad model for Twitter comes from Idealab founder Bill Gross, who launched Tweetup today. The startup ranks Tweets based on a proprietary algortihm (everyone's got one these days), and any brand manager/publisher can bid on placement in that ranking much like Google's AdSense. So far, it's only got $3.5 million in startup funding, but one of the moneymen is Steve Case, who told the New York Times today: "Tweetup is to Twitter what Google is to the Web."
Let's take a second to parse that statement. If Tweetup (not to be confused with TweetUp.org) is trying to be Google, but within the limited universe of Twitter, it would have to have as many proportional eyeballs as Google. Not gonna happen. Tweetup's reach is dependent upon how many publishers it signs up. So far, it's on board with Answers.com, Seesmic, BusinessInsider.com, Twitdroid, as well as some others, but it's not exactly Google-like reach. Revenue is split 50/50, so yeah, there's incentive, so we'll see how wide a net it can cast.