Google doesn't know what it is doing when it comes to social media.
They had a perfectly good landing site for people, iGoogle, that aggregated feeds that people look at daily, including Twitter, Facebook etc. They neglected it and then, without any in depth analysis or community research, shut it down. To "replace" it they launched Google+, the "next Facebook". Most of the iGoogle community then defected either to Facebook or to ighome.
For a social media site to become useful, it needs to reach a certain mass. After all, you aren't going to hang out on a site where it's only you or only you and one other friend. Facebook's "stickyness" is due to practically everyone and their Grandmother having accounts. And, despite the security issues (which Facebook has very slowly improved on), Facebook is good enough that the masses put up with it.
The only reason why Google+ has 2 billion profiles is because they forced everyone to sign up for access to other Google services. While this seems like a good way to reach critical mass, it's acts against the psychology of social media. Most people join social media sites because they want to, not because they are forced to. This breeds a certain amount of resentment against the brand.
It isn't that Google+ is bad or that it isn't somewhat useful, Google just went about it the wrong way. In my opinion, they created a strategy that would solve part of the critical mass problem, but completely missed the mark when it came to the social aspect.