For Facebook, buying Instagram was a steal. In a note this morning, Citigroup says that it has reexamined its valuation of Instagram and now believes that it is worth $35 billion. Citi had previously valued Instagram at $19 billion, that itself being far above the $1 billion that Facebook purchased Instagram for back in 2012. The increased value is due to Instagram's announcement last week that it now has over 300 million active monthly users — meaning that it's growing faster than expected. Citigroup also believes that Instagram could eventually contribute around $2 billion in annual revenue to Facebook if it were to fully monetize its network. Most of Facebook's revenue still comes from Facebook itself, but beginning next year, Citi expects to see Facebook begin tapping into its other apps and services a bit more.