No one but YouTube (1 billion monthly users but not all mobile) even comes close.

Facebook reported 890 million daily active users in December, up 18% year over year, and 745 million mobile daily active users, an increase of 34% year over year. There were 1.19 billion mobile monthly active users, an increase of 26% year over year. 

Here's a look at Facebook's monthly active users:

Facebook said last quarter that it would start monetizing products like Instagram, Messenger, and WhatsApp once each hit 1 billion active users. In the earnings call, Facebook reported that Messenger has 500 million monthly active users, Instagram has 300 million monthly active users, and WhatsApp has 700 million monthly active users.