I must say, i find Google+ rather disappointing. There's way too much to read, hard to go through every post without a proper search.
i.e. I have a lot of Googlers in a dedicated circle. I would think their posts would be interesting, business related etc. However the majority is personalized, and utter crap for those who don't really care for their out-of-office lives. So finding the gems in all their posts is way too difficult.
Same goes for SEO related material.
Instead, the people i do care about reading up on their out-of-office lives are not at Google+ at the moment, or still use FB for updates. Like i do myself.
So in conclusion, i think Google+'s downfall in the end will be the lack of public access due to the invite system. As a network, i'm just not triggered enough to A. participate myself (cause of the lack of real life friends on it) B. read up on other posts.