CIO — Facebook announced yesterday its much-anticipated location-based service, Places. Places, which is currently available only on mobile devices that support HTML 5 and with a GPS or geolocation feature, is set to pose a threat to other more-established services such as Foursquare, Gowalla, MyTown and Whrrl.

Here's what you need to know about the alternatives and how Facebook's four big geolocation rivals stack up.

1. Foursquare

Foursquare, the mobile social networking startup that launched in 2009, lets users share locations with friends and awards points,badges and other perks for checking in frequently. If a user visits a restaurant, for example, he'd check-in to that location via his mobile device. His location would then become visible to his friends on Foursquare.

Foursquare has also been praised for its privacy controls: Only Foursquare friends can access specific information about your whereabouts at a given time, and only if you choose to let them know.

Retailers are jumping on the Foursquare bandwagon, too. Clothing retailer Gap (GPS), for example, offered Foursquare users 25 percent off if they checked into that location. Ann Taylor offered a similar deal, too: 25 percent off Foursquare mayors (users with the most check-ins to that venue in the last 60 days) and 15 percent off to each customer after their fifth check-in.

2. Gowalla

Gowalla's service focuses heavily on travel and awards. With every check-in, users are awarded "stamps" for their passports—a log that tracks the places they've visited. They can also collect pins for "trips" they've completed. Some of these trips—through parks and historic walking tours, for example—are designed by sites such as National Geographic and CNNMoney. Gowalla users may also encounter "digital souvenirs" in their travels, which are redeemable for real-world rewards such as apparel, movie tickets or gadgets.

3. Whrrl

Whrrl users earn points and rewards for check-ins and by listing recommendations of places they visit, such as restaurants and bars. Users can earn additional points when people save, complete and share their recommendations.

MyTown

MyTown is a geolocation game centered on "buying and owning" your favorite real-world local shops, restaurants and hangouts. MyTown users check-in at locations to unlock rewards, then "buy and own" real-life places.

During the day, MyTown users "collect rent" when people check-in to their shops—the more people who visit your stores, the more it raises your property's value. Users can also cash in their game currency they've earned at MyTown's virtual store, where it's good for buying more buildings, content packs and more.