The official 105 million figure is a lot more than the 60 million that had previously been speculated.
This week, Twitter also announced it is launching its long-awaited advertising platform, Promoted Tweets, with big brands such as Sony Pictures, Red Bull and Virgin America becoming the first advertisers to sign up.
The micro-blogging site's new ad model will allow advertisers using Promoted Tweets to have their ads displayed at the top of some Twitter.com search results pages.