Twitter Circle
twitter.com
twitter.com
This is really helpful when your social media following grows beyond friends and family but isn't large enough to justify creating a completely separate private account. Whenever I post photos of my kids or other things that are only interesting to close friends and family, I can flip the switch to only share with a small group a pre-selected people. The other followers I've picked up along the way don't have to see that content and I can limit distribution. It's win-win.
I suspect it's more of an organizational tool problem than simply having a number to limit distribution, and that this is the lowest hanging fruit to create a solution for - for better or worse depending on your use case or needs; and externalized value no longer created by using this limitation may or may not result in a net benefit for you.
Or maybe an embedded twitter circle or something that is private to only those who loaded the embed? I don't know, just brainstorming but I think Twitter could definitely take on/over Disqus with something like this.
There is only one Circle per user.
Not sure why everyone seems to be thinking you can define multiple circles, a la email alisases or Google+