There are more than 500,000 businesses using Foursquare

"Two weeks ago, we passed a pretty big milestone on foursquare, with over 10,000,000 people in the foursquare community. Last week, we hit another one that excites us just as much: we now have over half a million merchants on foursquare."

Source:  Blog post by Foursquare, 6th July 2011