Facebook Inc. unveiled a service that makes it easier for users to share their whereabouts with other people, making a deeper push into a fast-growing market pioneered by Foursquare Labs Inc.

The new feature, Places, is designed to help users of the social network find friends and disclose their own location, Facebook said.

Services that help Web users share information on their whereabouts could generate as much as $4.1 billion in annual ad sales by 2015, according to Borrell Associates.