Some new stats have been publishes by a third party company called Twopcharts looking at Twitter. At last count, Twitter has about 974 million users, but it's less clear how many of those accounts are still in use. Twopcharts has offered up some numbers that claim twitter is much less active than the 974 million user number might suggest.
The stats that Twopcharts had gathered claim that 44% of those Twitter accounts have never made a single tweet. That means that 44% of Twitter's accounts have never been used. The company goes on to claim that only 126 million of Twitter accounts have sent a tweet in the past 30 days.
Twitter is tight lipped about its stats for the most part and doesn't like to talk about user retention. What Twitter has said in the past is that it has 241 million average monthly users as of December 31 2013. Twopcharts also claims that 391 million Twitter accounts have 0 followers and about 232 million haven't followed a single Twitter user themselves.