Facebook-Instagram Acquisition (April 2012): What explains the negative reaction of Instagram users to their acquisition by Facebook? - Quora
As I see it, Instagram users have enjoyed an experience that felt like being stuck on a small, hidden island. It was private, initially unknown and thus enabled new social connections with zero embarrassment. And it was also non commercial.
The first breach began with the launch of the Android app, which made Instagram less exclusive in the eyes of iOS users.
The vibe I catch from Instagram veterans is that their island is now sold to a large real estate company, which is going to start building skyscrapers there. Their main fear is probably the commercialization and the privacy settings that are now subject to changes.