Don't anyone find it amazing how little money they make per user (and still have a good business)?
The article doesn't say how much time the users spend on FB, but they say that users spend 100 million hours monthly on watching videos. If we assume that's 50% of the time spent on FB, users spend 200 million *365 = 73 billion hours yearly on FB or in average a little less than 50 hours per user. For $15.
So roughly speaking, Facebook makes $1 for every 3 hours a user spends.