Facebook has begun allowing users to search the Web from within Facebook, using Microsoft's Live search service.

The arrangement, first announced in July, offers a revenue opportunity for both companies. But they'll need to convince people to use the search feature, and it's uncertain they will in its current form.

"I'm not sure the experience they offer is optimal," said Greg Sterling, an analyst at Sterling Market Intelligence.

Still, the deal is a coup for Microsoft, which has been struggling to boost its search business. It might be the biggest search agreement for Microsoft in terms of potential users, Sterling said. Microsoft owns a stake in Facebook and has an existing exclusive agreement with the site for banner advertisements.

Starting Tuesday, Facebook users began to notice that when they start typing in the search bar on Facebook, a drop-down menu appears and they can choose to search Facebook or search the Web. If they choose the Web, results appear on a new Facebook page, with advertisements on the right of the screen.

At the top of the results list is a link to do an "Advanced search on Live.com," which launches a new window with results displayed at Live.com. Those results are sometimes slightly different from the ones on the Facebook page and include the ability to expand a search to include images, news, maps and videos.

The results are different because Facebook uses certain filters and doesn't display sponsored results, said Matt Hicks, a Facebook spokesman.

In addition to requiring users to do an advanced search to find image or map results, the results format looks different from other leading search providers, another potential drawback, Sterling said. Each item appears in a box that lists the Web site, a description of it and a link. "The presentation of results is a little strange, it's a little unfamiliar for search results," he said.

However, the companies may view this as the first try, with plans to add more capabilities.

"As we evaluate user feedback and results we'll explore additional ways to integrate Live Search more deeply into the Facebook experience," wrote Angus Norton, senior director of Live Search product management at Microsoft, in a blog post. He called today's service a first step.

In the future, Facebook should look for ways that search results and other content on the site can be more relevant to people in the context of their visit to Facebook, said Jeremiah Owyang, an analyst with Forrester Research. "This is a long-term play," he said.