Anecdotal evidence: I had never messed with Instragram until I got a girlfriend whose peer group used that exclusively over Facebook. These are all in the 28-32 age range. It seems they were all eager to use a system that just focused on sharing photos and didn't want all the 'bloat' associated with Facebook. In addition to preferring the focus on photosharing, there was a sense of just wanting to be back on a networ…
It is amusing that there's less friction to move to a new network than to clean up the old one. Unfriending (or hiding/muting/whatever while still actively using Facebook) is much more likely to be taken as an affront by your long-time Facebook friends than simply stopping using FB much. But eventually it seems like the same people will move to the new network of choice, and try to contact you there, and you've got t…
So in my experience I've found that people follow me but because the tool doesn't very easily show if someone follows you back, without digging back and forth through your followers, it's very easy to just unfollow the friends who don't provide content I care about.
It's more similar to Twitters model of pub/sub vs facebooks 1 to 1 dynamic. If I 'defriend' on facebook it forces you to defriend both ways right? I think facebook has made attempts at fixing this with subscriptions but ...