Xmarks, a service that lets users synchronize their web bookmarks across browsers and devices, today announced the first big piece of its plans to make money — an advertising service it calls SearchBoost.

The San Francisco company is already using the data collected from its 4 million active monthly users to enrich the search experience. When users perform a web search with the Xmarks browser add-on installed, they see both a star rating and a category ranking. For example, you might see one search result that is the most popular among Xmarks users in the technology category, while another is ranked tenth, so you’d probably click on number one first.

If an advertiser has a SearchBoost account, that same data gets shown to Xmarks users in their search engine ads. If someone did a search for “hiking shoes” and an ad for REI ad showed up, they’d also see that REI is ranked number one for outdoor gear. Or if an ad for Zappos shoes showed up, they’d see that Zappos is ranked number one for shoes. Chief executive James Joaquin said that in a test conducted with 200,000 Xmarks users, the presence of SearchBoost ratings increased the clickthrough rates on ads by 15 percent.

Of course, the improvement probably depends a bit on your rating — if an advertiser only has 2 out of 5 stars and is ranked 40th in a category, they might not want to broadcast that information. Joaquin acknowledged that the boost mostly works for companies who are ranked in the top 25 or so in a category.

“If you have a low rating you are going to self-select away from this product, and we’re okay with that,” he said. “We’re highlighting the better, higher-quality advertisers.”

The SearchBoost product also includes an analytics service, so advertisers can see which campaigns are paying off — both for themselves and for their competitors. Pricing starts at $29 a month, and Xmarks is also offering a free 30-day trial.

Xmarks launched some of its features last year at the DEMO conference co-produced by VentureBeat.