In my opinion Instagram worked because it was at the right place at the right time.

Radar was a photo sharing app that I loved using in 2008-2009. You took photos of what you were doing, edit them very simply, and shared them with your community of friends. The problem with Radar is that it went quietly in to the night. I don't recall the exact reason it shuttered, but I think they made some poor choices in features that killed it.

I always find it interesting to read posts about the start of Instagram and never to find references to Radar.

For Reference: http://readwrite.com/2009/01/27/radars_photo_sharing_app_com...