I went to have a look and it looked to me like Google's attempt to woo people away from Facebook by creating something similar. Here's an interesting comment on it:
Half an Hour: The Google Ecosystem:
This is an illustration of the Google Plus Ecosystem I created to try to explain the flow of information through Google Plus from its (currently undocumented) sources through to its (currently broken) output.
One of the problems I have found with this "me too" approach to designing social networking sites is that it is counterproductive. Initially there were improvements.
First there was Geocities, which tried to group web sites according to themes and common interests, and promote interaction among the webmasters. Then it was taken over by Yahoo!, which didn't understand the principle, and killed it.
Then there was SixDegrees, which was real social networking, but before its time. The graphics loaded too slowly on the dial-up connections that most people used back then.
Then there was MySpace, whose main drawback was that it was designed for (and possibly by) celeb-following 11-year-olds with its garish graphics.
Then came Facebook, which was originally for undergraduates, and appealed to many with its clean, minimalist approach. When it was opened to the hoi polloi it became a useful place to keep in touch with friends, family, acquaintances, work colleagues and the like, though it also had the problem of people collecting "friends" like some people collect postage stamps, but indisciminately. It also became less useful when it branched out into third-party "apps", which often competed with each other, and dispersed the effort.
For example, you could have an app that linked to your favourite books and what you were reading. The problem was that there are about six other apps that do the same thing, and when you are using App A and your friend is using App B, then to compare books you have to enter all your books all over again in App B, very often with a clunky user interface. So I have a general rule of "No more apps". If anyone invites me to anything on Facebook, and it has a rigmariole about asking my permission to access information about my friends, I click "Cancel" and go no further. And if I want a site to compare books and what I'm reading with my friends, I use one that does it well, like Good Reads. From there I copy my reviews of the more interesting books to one or other of my blogs, and from there an announcement filters through to Tumblr or Twitter to Facebook, so my Facebook friends can see what I've been reading, without using clumsy "apps".
But with Google+ the problem is likely to be exacerbated. Soon one will have one set of friends on one social networking site, and another set on another social networking site, and one will need a metasocialnetworking site to bring them all together in one place.
Yahoo! recently dropped yet another of its useful services (MyBlogLog), and urged people to join Pulse instead, which is their attempt, like Google+, to compete with Facebook. Instead of doing what they do well, they prefer to do what other people do, badly.