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Facebook Mobile User Counts Revealed: 192M Android, 147M iPhone, 48M iPad, 56M Messenger

Facebook keeps user counts for its mobile apps hidden, but researcher Benedict Evans found a way to uncover them and they provide critical insight into the direction and performance of Facebook's mobile efforts. Most interestingly, Facebook's Android user count is growing much faster than its iPhone user base, but is found on a lower percentage of Android devices.

http://techcrunch.com/2013/01/04/how-many-mobile-users-does-facebook-have/