The Plentyoffish app lets you check your inbox messages and view your matches. It also uses some location-based technology to show you matches in your area.
One app you might not be familiar with is Flirtomatic, which is part social network and part dating site. More popular overseas than in the United States, Flirtomatic lets you "flirt" with strangers (or friends) via either the dedicated mobile app or the mobile Web (for the non-smartphone users who also deserve love). You make a profile, and instantly you're connected to thousands of other profiles. The service seems more like a fun way to waste time than to find true love, but if you're seriously looking for your soul mate, you can search for people near you who share similar interests.
But be cautious. That sweet guy with the big green eyes who loves puppies as much as you do could turn out to be a one-armed ex-con who has never met a puppy he didn't hate. When meeting people on one of these services, make sure you arrange to go to a public, well-lit place, and alert your friends in case you need to bail.
Location-based apps that allow you to check in to a bar or restaurant, like Foursquare or Gowalla, are all the rage right now, especially since Facebook has jumped on the bandwagon. They can help friends find each other during a night on the town, and they're useful for sharing tips about a hot-drink special in the neighborhood.
But these apps are also a good way to keep tabs on your partner. Facebook even allows users to check in the people they're with at a given location, indicating not only where they are but who they're with. So if your girlfriend just checked into the Slanted Door with her ex-boyfriend, you should probably check up on that.
I've also heard of people using these apps to avoid exes. You're about to hit up the gym decked out in your grossest sweats, but--oh, wait--your ex-husband and his new girlfriend just checked in. Better wait until after dark to avoid an awkward confrontation.
Facebook will also send you a notification on your phone when one of your friends checks in at the same place you've checked in. This is nice because if that hot girl from your bird-watching club checks in to your grimy local dive, you may have a small window of time to sober up and brush your hair.