This is why I loved Instagram and Twitter, it wasn't Facebook... where people I don't even know anymore are all my best friends.
I wonder what sort of people putting photos on Instagram weren't also sending them to Facebook. Wouldn't it likely be people who don't/won't use Facebook anyway?
Friction is my big guess; Instagram is effectively a drop-in replacement for your smartphone camera, with some extra features that are incredibly popular right now (e.g., the overused look-ma-its-the-70s effect).