Like an ancient riddle, the secret to making money from Internet social media has obsessed, and frustrated, Web entrepreneurs for years.

But Bill Gross is not your ordinary entrepreneur.

As the founder of Goto.com in the late 1990s, Gross created the first successful implementation of  paid search, the concept that today underlies Google’s roughly $24 billion-a-year cash machine.

His latest project, TweetUp, aims to do something similar for Twitter messages by ranking Tweets by relevance and by allowing Twitter users to bid money for their messages to rise to the top. (Under the bidding system, a person selects keywords and bids a penny, a quarter, or more, for their Tweets to appear on searches for those keywords).

billGrossBy creating a marketplace for Tweets, Gross contends that he can provide Web publishers with custom-tailored feeds of highly relevant Twitter messages about various topics and enhance a Web site’s content. A financial news site, for instance, can integrate a live feed of Tweets to accompany articles on various subjects, like a particular company’s stock or a news event (Gross says TweetUp will split the revenue 50/50 with publishers).

But TweetUp, which launches on Sunday night, will require a big shift in the dynamics of online social media to succeed – it’s effectively instituting a price of admission for user generated content.