Retro photo sharing app adds another 1m in November.
The founders told Venturebeat that their small company – just seven employees – just keeps on growing, having hit 12m users in its first year to October 2011.
Co-founder Kevin Systrom said the viral effect was doing most of the work.
He said: “We’ve always been growing fast, but I think network effects help — we’ve got millions of people who love Instagram and they spread the word for us."
Instagram is an iPhone-only application, although its API is taking it elsewhere. However, Instagram did say an Android is in development, but there's no launch date to report.
In August the company confirmed its users were sharing 15 new photos every second.