Instagram January 2013: 90 million monthly active users.
Adam Rifkin stashed this in Active Users
Stashed in: Instagram!
90 million monthly active users.
40 million photos posted daily to the service.
8500 likes per second.
1000 comments per second.
Facebook stats for comparison:
1 billion monthly active users.
37,073 combined likes and comments per second.
Wow. Considering that Instagram is mobile-only and has less than a tenth the active user base, the fact that Facebook only has 4 times the engagement is very impressive for Instagram.
As of February 2013: 100 million active users.
The Merc writeup of it:
Instagram blog post: http://blog.instagram.com/post/44078783561/100-million