For some time I have wondered why Facebook isn't used for public sharing in the same way as Twitter. I view Twitter's generally public nature as its main strength. Most Twitter profiles are public. Following someone allows weak, one way relationships to be formed. I saw no reason why Facebook could not implement such functionality on top of their existing friend model and kill Twitter off.
Google+ has answered that question for me - its too complicated.
A quick rundown of how Google+ works is in order -- note I've only just started playing with it so I hope I'm not misunderstanding its sharing/relationship model.
Anyone on Google+ can add anyone else into any of their Circles. This forms a one way relationship akin to a Twitter follow. If the person followed also adds the follower to one of their Circles then the relationship becomes "mutual". This may seem similar to but it is not identical to friending someone on Facebook.
The difference appears when it comes time to share. Let's use a more concrete example.
Alice and Bob work together. Bob adds Alice to his "Friends" circle. Alice puts Bob in her "Colleagues" circle.
In the Facebook model they would be considered friends, and the default sharing permissions would mean that they both see all of each others posts. It is possible to set permissions on a per-post basis in Facebook but I'm dubious that many people actually do this.
The default in Google+ is to share to "Your circles" which is more or less equivalent to the Facebook default -- anyone in any or your circles can view the post. However the sharing interface on Google+ makes the permissioning more in your face so there's a higher likelihood that people may make use of their Circles to limit visibility of their posts.
Let's say Alice posts to her "Friends" Circle -- Bob won't see this because Alice has put Bob in her "Colleagues" Circle. Coming from the Facebook friend model this would seem a little weird since their relationship is mutual in the sense that each of them is in one of the other person's Circles.
However this model allows for Twitter style public sharing. Bob can put Alice in any of his circles and will see all of her Public posts regardless of whether Alice wants to recognise any relationship on her side. Note that Alice still has the option to block Bob but the important point here is that one way follows are allowed in Google+. They are actually sort of possible in Facebook but its a pretty weird edge case.
I think its pretty clear now why Facebook hasn't merged Twitter style public sharing. Its too damn complicated.
The Facebook mutual friend relationship is easy to understand. You are either friends or you are not. When you share you are sharing with all of your friends (most of the time).
Twitter's follow model is also easy to understand. You can follow people you like, they can choose if they want to follow you back or not. When you share you share publicly (most of the time).
Google+ sits in a weird in-between model that is fine grained but complicated.
I can see where they are coming from. In the physical world your social circles are separated by, well, physics. When you're with work people that's your work context and you share appropriately (some don't, hilarity ensues). When you're with friends or family you share other, probably more private, stuff. Whereas posting to Facebook is essentially sharing with ALL of those groups at once Google+ Circles attempts to help you keep them separate.
Sure, some people do struggle with over-sharing on Facebook and I've heard from a few that they don't post as much as they would like to because its tedious to segment their friend list. But a solution which involves conscious thought both in managing friend lists AND who should see what during posting is just too complicated. For this reason I suspect that Google+ will struggle with adoptance.
Sometimes, despite the accompanying limitations, simple is best.