Social Networking, the Dark Side
The powers of social media can be used for evil as well as for good:
Unfortunately, as PleaseRobMe points out, updating Foursquare on where you are implicitly updates Foursquare on where you're not -- at home.
On the other hand, with so many people doing this kind of thing, you have what amounts to an information flood. If you're already at risk (being stalked, for instance), I can see the immediate danger. Otherwise? It's never been that hard to figure out that a house is empty.
