A user in another thread aptly described it as Twitter with pictures. I don't use Instagram and don't really see a reason to, but that doesn't mean others feel the same way.

Roughly speaking, you might say:

    Facebook:  Social with everyone      (polysocial?)
    Path:      Social with close friends (oligosocial?)
    Pair:      Social with your S.O.     (monosocial?)
    
    Twitter:   Microsocial in text with everyone
    Instagram: Microsocial in pictures (nanosocial? :P)

Having said that, I actually had a recent discussion with someone about how the heck they were going to monetize the service, and we didn't reeeally come up with a good suggestion aside from the expectable ads and premium filters. Instagram: Microsocial with everyone