Foursquare, the location-based check-in service, has been steadily expanding since it first hit the Web in 2009. The company who recently announced that they have 10 million users has hit another major mile marker — over 500,000 merchants have joined the service.

Merchants have major benefits when they sign up for the service. They can create specials that help promote their business through the network, they can see important statistics like which customers are most loyal and they get to manage their own venue page so they can make sure users don’t create a listing with incorrect information.

The merchant program may have just reached a major milestone with over half a million members, but we can expect to see even more merchant accounts in the coming months.

Foursquare has been on a steady climb to success and June was a big month for the service. They recently closed a round of funding where they raised $50 million. The new funding will be used to expand their merchant options, make new hires and work towards international expansion. The service rolled out a much needed update for their iPhone app that made check-ins faster than before and they also announced a partnership with American Express that allows users to link their card to Foursquare and get cash back.