1. Email Statistics Report, 2011–2015;Radicati,2011
2. Recommendations in location-based social networks: a survey
3. Instagram Hits 300 Million Monthly Users to Surpass Twitter, Keeps It Real With Verified Badgeshttp://techcrunch.com/2014/12/10/not-a-fad/
4. This Chart Shows How Instagram Reached 150 Million Users in Half the Time of Twitterhttp://www.businessinsider.com/instagram-growth-chart-2014-2