Location-based social network Foursquare counted its 40 millionth check-in 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, we clocked Foursquare’s total check-ins at 22 million. In short: Foursquare appears to have doubled its check-in 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 the 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 — why would new users join Foursquare if their existing social network provides the same experience?

Alternatively, Facebook may offer a location platform: That might let Foursquare, Gowalla and their ilk build upon Facebook’s massive userbase, rather than getting squished like a bug.

Tags: foursquare, location, million, twitter