Facebook Inc. (Nasdaq: FB), owner of the world’s most popular social network, is testing an advertising service that lets companies cast a wider net by placing more promotional messages in users’ News Feeds.

Under the program, companies can place ads in users’ main feeds on smartphones and personal computers even if those members or their friends haven’t signaled they like the advertiser, said Annie Ta, a spokeswoman for Facebook. Until now, these types of ads only showed up on PCs to the right of the Facebook News Feed, which displays activities by friends and companies or groups a user has “Liked.”

Facebook is grappling with investor concerns about its growth prospects as more users access the service from wireless devices, where its advertising options have been limited. Facebook rolled out its first mobile ad service for companies earlier this year with “Sponsored Stories,” enabling companies to pay for content in the News Feed after members or their friends have indicated they like a company.

The new test service includes limits so that a member’s News Feed isn’t filled with too many advertisements, Ta said. The test is the company’s third major effort in mobile advertising. Earlier this month, Facebook unveiled a tool that helps developers advertise their wares.

Facebook, based in Menlo Park, California, fell 5.7 percent to $20.38 at yesterday’s close in New York, the biggest decline in two weeks.

The stock has dropped 46 percent since its May 17 initial public offering.

The website Inside Facebook earlier reported the test of the service.