Is Facebook working its own in-site Foursquare killer? That seems to be the loose consensus ahead of tomorrow's announcement. The social network sent out invites to a number of outlets for a "news" event at its headquarters, tomorrow at 4:30 PM PT.

According to the invite, "During the event, Facebook representatives will provide an update on the service's features and products." Most of the speculation about the announcement centers around the possible launch of new location-based services for the social network.

"The new feature–the development of which has been the biggest open secret in the digital space–has been a long time coming," Kara Swisher writes over at her BoomTown blog, "as Facebook has noodled on how to incorporate the hot trend that was pioneered by Foursquare and other start-ups."

How big of a blow would such a feature be to a service like Foursquare? This may not be entirely bad news for the startup. According to Swisher's sources, "Facebook's service is likely to integrate other third-party services and will give developers on Facebook's platform geo-location capabilities."