Instagram has 13m users and 7 employees

"The popular photo app that’s responsible for flooding the internet with fuzzy, low-contrast, retro images of cats, hipsters and urban street scenes shows no sign of slowing down.
Of course we’re talking about Instagram and its iPhone-only app for sharing stylized photos. The startup attracted 12 million users in just over a year on the App Store. By early November, the small company — and we mean small; Instagram has just seven employees — grabbed another million and now boasts well over 13 million application users, co-founder Kevin Systrom shared with VentureBeat."

Source:  VentureBeat, 18th November 2011

Earlier - 10m users announced in September