SAN FRANCISCO — Bill Gross, the serial entrepreneur who pioneered search advertising, is unveiling a venture today that aims to make money by allowing people using Twitter to bid on key words to give their posts top ranking.

Called TweetUp, the service will also organize the posts according to their popularity as measured by how often readers repost them and click on links they contain.

Gross said he had signed deals with other outside Twitter services like Seesmic, TwitterFeed and Twidroid to display TweetUp’s rankings. A TweetUp search bar will appear on Web sites like Answers.com and BusinessInsider.com. TweetUp will split revenue evenly with each partner, he said.

A goal and a gamble

The goal is to cut through the clutter of thousands of irrelevant posts on topics of interest and keep the useful ones from disappearing into a torrent of messages.

The gamble is whether Twitter users who have turned the microblogging service into a global communications phenomenon will be willing to pay to get their 140-character messages noticed — and whether other Twitter users will view such paid placement as legitimate.

“We feel Twitter is unbelievably powerful, but finding the thoughtful Tweets amid all the noise is unbelievably hard,” said Gross, founder of Idealab, a technology company incubator based in Pasadena, Calif. “What we’re bringing is a new sort-order to Tweets.”

TweetUp has raised $3.5 million from a group of investors that includes Index Ventures and Revolution, the investment firm of Steve Case, the former chairman of AOL Time Warner.

Case said he was persuaded to invest in TweetUp by Gross’ role in developing paid search at GoTo.com, an Idealab spinoff, in the late 1990s. Advertisers bid to have their listings placed at the top of search results, a controversial practice at the time. Yahoo acquired GoTo.com, which had been renamed Overture, for $1.6 billion in 2003.

“TweetUp is to Twitter what Google is to the Web,” Case said.

How it works

Here is how the service will work, according to Gross: People can bid on key words or phrases, like “iPad” or “solar energy,” to push their Twitter profile or posts to the top of TweetUp’s rankings. Bids begin at 1 cent, and people will pay each time their profile or a post shows up in a search.

Gross stressed that bids were not required to appear in search results. The service will also calculate rankings based on an algorithm that uses data from a company called Klout that measures a Twitter user’s influence. Bit.ly, a service that shortens Web addresses for display on Twitter, will provide data on how often people click on a link in a post.

So who does Gross expect to pay to put something as ephemeral as a Twitter post on top of the charts? “I think everyone who is looking to build a following will pay,” he said. That means companies that want to build their brands as well as individuals who hope to drive readers to their Web sites.