From a Facebook Blog Post:
…in the last two years we built the Facebook onion site and onion-mobile site, helped standardise the “.onion” domain name, and implemented Tor connectivity for our Android mobile app by enabling connections through Orbot.
Over this period the number of people who access Facebook over Tor has increased. In June 2015, over a typical 30 day period, about 525,000 people would access Facebook over Tor e.g.: by using Tor Browser to access www.facebook.com or the Facebook Onion site, or by using Orbot on Android. This number has grown – roughly linearly – and this month, for the first time, we saw this “30 day” figure exceed 1 million people.
Read the Complete Blog Post