foursquare-logo1 Location-based social network Foursquare counted its 40 millionth checkin a “couple days ago,” according to a tweet from one of its developers.

The number shows that Foursquare’s growth rate is accelerating: Five weeks ago, the company announced that the total number of checkins had reached 22 million. In short: Foursquare appears to have doubled its checkin rate in just over a month.

It’s just over two weeks, meanwhile, since Foursquare hit the million user mark. What’s more, recent acquisition rumors have driven Foursquare hype to new levels.

But Foursquare may soon face a new challenger. Facebook is launching its location features this month. Depending on how Facebook implements location into its service, Foursquare may see its growth stall. After all, why would new users join Foursquare if their existing social network provides the same experience?

Alternatively, Facebook may offer a location platform that lets Foursquare, Gowalla and their ilk build upon Facebook’s massive userbase, rather than squishing them like bugs.