"...fake users on Twitter may outnumber the real thing by more than 2 to 1."

This sounds about right to me: 360 million fake accounts, 140 million real accounts.

And of those 140 million real accounts, most of them are readers, not writers.

Yes. This makes complete sense.

Twitter can still make a lot of money advertising to those 140 million readers.

Twitter makes a lot of money advertising to those 140 million readers.