I think the point that everyone is missing here, is how heavily integrated Instagram is in non-Facebook social networks (i.e. twitter and Flickr). Yes, part of the valuation was due to Instagram being an up and coming social network. But the main value of this acquisition comes from the fact that, with this single purchase, Facebook is able to leverage multiple social network's userbases into their own.

From captain Zuck's post on Facebook "We think the fact that Instagram is connected to other services beyond Facebook is an important part of the experience." (https://www.facebook.com/zuck/posts/10100318398827991)