1. Foursquare website, 2013. https://foursquare.com/.
2. J. Marshall, Google launches location-based mobile ads, 2010. http://www.clickz.com/clickz/news/1725773/google-launches-location-mobile-ads.
3. Loopt website, 2013. http://www.loopt.com/.
4. Brightkite website, 2013. http://www.brightkite.com/.
5. Google’s latitude, 2013. http://www.google.com/latitude.