Revenue hasn't been a problem, Twitter will do over $2 billion this year. Wall Street has been fixated on user growth though. Probably thanks to Facebook touting its ever increasing user base.
It is still growing, but the momentum has been slowing for over a year. I googled, "twitter user growth" and every result points to disappointing user growth
Well at some point growth has to slow. There are a finite number of people interested in what Twitter has to offer.