People have been asking for a long time to find friends who like things, etc.

FORBES: LIVE: Facebook Unveils Graph Search--Its Long-Awaited Internal Search Engine

Foursquare emerged at the South By Southwest conference in 2009 with its check-in service for people to share find where their friends are in real-time.

FORBES: Foursquare Wants To Own Offline Loyalty, Local Recommendations