Every ad I see makes Twitter maybe 5 cents? I'm guessing they earn $40 / year on me (I'm not a heavy uesr). I wish I could just pay them $40 and not have any ads inserted into my stream for the rest of the year.
Facebook's per-user revenue in the US as of Q4 2015 was around $11/yr (source: http://www.businessinsider.com/facebook-average-revenue-per-...) although its grown by 50% yoy.
Chances are if FB / Twitter / Instagram had high confidence that could monetize their product with a subscription model averaging even $10/user a year I think they wouldn't hesitate to implement such a model.
I think comparisons with Facebook are a big part of Twitter's problem. They aren't Facebook, never will be, and shouldn't try to be. If they use Facebook numbers as a benchmark, then they are totally screwed.