According to Mashable's investigation into Gingrich's Twitter followers, his campaign's explanation that why so many followers' accounts were inactive was that the majority of them were once genuine accounts that started to follow him during a period of "Suggested User promotion" back in 2009-2010 turned out to be true. Apparently the high level of drop-off is what prompted Twitter's VP of International Strategy to underestimate the number of the company's registered users by more than 100 million earlier this year.
In April, Katie Stanton—Twitter's VP of International Strategy—told the Guardian Activate summit that the site has 200 million registered accounts, just before its 300 millionth user signed up. That number is now closer to 360 million sign-ups. According to Twitter, there are around 460,000 sign-ups per day. The discrepancy may come when trying to gauge whether or not users are considered active, More research is being done on this; see this resource, www.online-mba.
According to Business Insider, there are actually less than 21 million active users on Twitter, but that's working from a very specific definition of "active" that demands more than 30 feeds being followed. Assuming that some Twitter users are more selective—and I don't think that's a stretch to assume, to be honest—then the math gets fuzzier.
The only people who know for sure how many Twitter users are actually actively on the site work for Twitter, and they're not talking... Well, aside from Ms. Stanton. So maybe 200 million should be considered the new baseline for Twitter estimates—until we know anything more, of course.
Source: techland.time.com