Start off exploring these main players in location-based services and then, when you feel comfortable with your marketing strategies, you can branch out to other smaller, niche services.

Foursquare With over 10 million members, foursquare is easily the most popular of all the LBS platforms. The system features check-ins, tips, badges, and points for accomplishments, a leaderboard, the most comprehensive specials platform, and a way for users to explore nearby tips.
Gowalla The Gowalla platform is dedicated to exploration and storytelling. Users check in to places and collect stamps and virtual items along the way. Users are then given a pin to show that they completed the trip.
SCVNGR SCVNGR lets you set up elaborate scavenger hunts. Aside from traditional scavenger hunts, it's ideal for creating a series of activities that act as a template to show users how to experience your brand. SCVNGR also has a rewards platform that allows you to give users incentives to do these tasks.
Yelp Yelp helps people find great local businesses like dentists, hair stylists, and mechanics.
Facebook Places Facebook Places allows users to share where they are, connect with friends, and find local deals. Any of Facebook's 650 million users can use Facebook Places.