1. Foursquare,
https://foursquare.com/
2. Yelp,
http://www.yelp.com
3. Gowalla,
http://gowalla.com/
4. Indvik, L.: Foursquare Surpasses 3 Million User Registrations,
http://mashable.com/2010/08/29/foursquare-3-million-users/
5. O’Dell, J.: Foursquare Day Sets Record with 3M+ Checkins,
http://mashable.com/2011/04/20/foursquare-day-2/