Considering that Facebook has eight times the number of users as Instagram has, keeping that growth will be a very hard thing. Facebook has 1.1 billion users. But for both Systrom and Instagram co-founder Mike Krieger, it really doesn't matter which of the two comes out on top since they both are part of the same company.
Perhaps Krieger had the best response to the question of whether Instagram could catch up to Facebook. "I don’t know--that's a good question. I think both of them are basically limitless in size, so maybe one day they’ll be the same size?"
source: FastCompany via BGR