April 2015: User Retention Rates on Top Mobile Apps Facebook, BBM, WhatsApp, Instagram, WeChat, Snapchat, Twitter (via Ryan Sarver)
Adam Rifkin stashed this in Active Users
Actually, this chart comes courtesy of @theinformation :
I found this chart via Ryan Sarver of Redpoint: https://twitter.com/rsarver/status/587655594464579585
Notice how Facebook stays at (or above!) 100%.
Josh Elman says that Snapchat's would look similar if the data sample was sub-25 and U.S. only.