They have to turn a profit. Look at how over-run youtube has become. It's pretty annoying, but they did it gradually which helped silent the complaints.
Twitter will do the same. Slowly but surely making it lamer than it already is. Look at its profit forecasts (from wikipedia):
"Some of Twitter's documents covering revenue and user growth were published on TechCrunch after they were retrieved by the hacker, Croll Hacker. These contained internal projections that in 2009 they would have revenues of $400,000 in the third quarter (Q3) and $4 million in the fourth quarter (Q4) along with 25 million users at the end of the year. The projections for the end of 2013 were $1.54 billion in revenue, $111 million in net earnings, and 1 billion users. No information about how Twitter plans to achieve those numbers has been published. Biz Stone published a blog post suggesting legal action for revealing the details was a possibility."\
I don't see how they can ever reach such high estimates no matter what they do. But they will certainly try.