In its 5th year only, it has reached 400mn MAU - monthly active users - and is officially bigger than Twitter. 75% of its users come from outside the US. Instagrammers create 80mn photos and 3,5bn likes daily. Some predict that in its first year, served through the Facebook advertising editor, Instagram can make around 600mn USD with ads (including video ads in a TV-style 30 seconds format), and surpass the billion in the year after.
Also see a comparison between Twitter and Instagram growth: