I think more than facebook, it’s twitter who need to copy the circle feature from Google+. Posting different things to different people is much more required on twitter than on facebook. I’ve my professional contacts and personal friends on twitter, I would like to share different things with them, without them knowing it.
Twitter could do it by modifying twitter lists to some bit. If I could make private lists and put people in them and share different things with different lists, that’s what I want. The problem I see, in implementing this is, everything is public on twitter, so eventually everyone can see all you updates. But then, that’s ok. Only that I don’t want to spam my follower’s timeline with tweets he might not be interested in and there’s no problem if he explicitly come on my profile to see all of the tweets I made.
I think sooner or later, twitter would need to implement this feature. It’s a much needed one. Google+ is a big threat to them, I believe.