Check-in services are social-networking apps that let you "check in" to tell your friends or the whole world where you are at the moment. One in five smartphone owners uses them.

These applications, such as Facebook Places, Foursquare, and a new one called LoSo, by a West Chester start-up, use the sometimes-controversial location wizardry of smartphones to shout where you are. This can be good, as when friends are trying to decide who's in the neighborhood and where to hang out for the evening.

And it can be bad, as when you let the burglar know you're at the airport on your way to Istanbul.

Facebook Places, which the social site introduced last year, is integrated into the Facebook app for iPhone, Android, and other devices. From the menu screen tap the "Places" icon to see which friends have checked in anywhere recently. If they're nearby, you can consider looking them up.

To check yourself in, tap the "Check In" button on either the Places or News Feed screen. Say you're in a coffee shop or park; if someone has checked in there before, the name of the place will appear on your screen, along with other locations in the vicinity. Otherwise, you can add the location, say something about what's going on there, and post a photo if you wish.

Foursquare, also for multiple devices, adds bells and whistles that identify user-recommended restaurants, shops, and entertainment nearby. The app makes a game of it by awarding points and "badges" for checking in. It'll name you the "mayor" of a location for checking in from there the most. Many businesses offer specials to people who check in at their locations.

LoSo is being rolled out this month, beginning in the Philadelphia area, by LoSo Inc. chief executive Rich Rodgers. It draws feeds from Facebook and Foursquare but focuses strictly on helping users find the liveliest restaurants and bars.

"The idea is we'll show you everything that's near you, but it's not only what's near you but what's going on in real time," Rodgers says.

LoSo enlists restaurant and bar operators to offer loyalty rewards and post what's being served, what's happening, what bands are playing. Going the other services one better, LoSo is set up to post user videos.

The nearly 20 percent of smartphone owners using location-based social networking are mostly 18- to 34-year-olds, according to  ComScore Inc.

It can be great fun, but remember that websites such as PleaseRobMe.com and RobMeNow.com warn people to think twice before revealing their locations, basically for the same reasons you ought to put timers on lights when you go away on vacation - because being too obvious about where you are, or aren't, can jeopardize your privacy or your property.

Contact staff writer Reid Kanaley at 215-854-5114 or rkanaley@phillynews.com.