Instagram has 300 million monthly active users

"We’re proud to announce that there are now more than 300 million Instagrammers. Over the past four years, what began as two friends with a dream has grown into a global community that shares more than 70 million photos and videos each day. Instagram is home to creativity in all of its forms, a place where you can find everything from images of the Nile River to the newest look from Herschel Supply or a peek inside the mind of Taylor Swift.
We’re thrilled to watch this community thrive and witness the amazing connections people make over shared passions and journeys."

Source:  Blog post from Kevin Systrom, 10th December 2014

Earlier - 200m announced on 25th March 2014

Note - This now means that Instagram has more MAUs than Twitter, according to most recent figures from each
Note - This argues that you shouldn't only look at MAUs to compare Instagram and Twitter