Question

You can use ________ services such as foursquare, gowalla, and brightkite to specifically locate friends or allow friends to locate you.

Answer 1

You can use location-based social services (LBS) such as foursquare, Gowalla, and Brightkite to specifically locate friends or allow friends to locate you.
These software-level services use location data to control features.
The information about the location is provided by using real-time geo-data from a mobile device or smartphone.