Twitter, Facebook, Foursquare, and others include the latitude and longitude of your location (if you have location posting enabled) with your social updates. The latitude and longitude can easily be converted to a real address by using Google or Geocoder. Google Street View also accepts these coordinates so strangers can actually view the location from where the social update was sent.
This data is available programmatically via APIs published by these services. This makes it possible to automatically search for keywords such as "at home". There is a demo of exactly this at http://www.weknowyourhouse.com.
How to protect yourself:
1. Turn off location posting
2. Don't check-in from home or a dorm room or other private spaces
3. Be careful with what you choose to share online