Twitter claims something like 2-300MM MAU. Which I just struggle to buy. I'd consider myself fairly well connected in a segment/industry that should be among the primary adopters of Twitter - and very very few are actually active on Twitter. Most are "active" in the sense that they click on links to Tweets.
I wonder how many unique (non-bot) users actually post on Twitter each month. I reckon that number versus MAU is going to be very asymmetric.