Facebook already has a conceprt of groups. I frequently share my updates/pictures with only to select set of groups. It's just that it is a bit hard to use it. Nothing that facebook can't fix. Don't know what is so great about this.
Things I want to share with friends of family may not be things I want coworkers seeing. I don't want to share photos of drunken antics with bar friends with my grandparents. I don't want updates about a sick relative to become office gossip.
Yes, the feature sort of exists in Facebook to kind of get this functionality, but there is a massive appeal in a network that treats the idea that not all acquaintances are equal as a core concept. And because it's a central value of the network, I have a lot more faith that it should actually work than the Facebook model, which has already failed me plenty of times in the past.
Hopefully, with Google+ Circles my private categorization of my friends is not shared with them. They should only see what I share.
(Granted, it's not very discoverable)
Only bummer is that you can't do fancy options like that with the Facebook iPhone app. You can only do your default, the normal options like Friends Only, or specific groups.