Google Plus is getting a lot of love from the early adopters but as far as a mainstream tool, I don't see the move happening with what's being offered now.
Features alone don't make for a social media transition... the experiences needs to differ. Google is making a stab at this with Circles which do allow for a much greater segregation and filtering of people and as users get more savvy about privacy, and more conflicted over who to let into their network, that's increasingly important. However that feature is in it's infancy [obviously] and getting usage of it from a mass market will not be simple. Google needs to increase the power and at the same time paint a picture of circles to make them a focus... But that can't be the whole show.
Facebook's advantages on the other hand are clear. 750 million users which means mom, jenny and your high school friends are all there. While this actually gives circles a better edge up [talking to everyone via Facebook is clearly not ideal] it also makes it very hard for Google to break free -- in order for me to stop using Facebook Google would need the vast majority of my network to move over, and that has to happen very quickly for most of them to stick around. Facebook, while very simple, also looks more "fun" than Google. Google has always been about absolute simplicity in their layouts and that leaves their sites feeling half way finished, even if everything works.
Add on to it Google's desire [need?] to implement their existing features. In some cases this is great... want email, they have that. But it's also a drag and can limit the doors by forcing a subpar or limited tool into the mix.
Google faces the adoption time issue. It's nice to converse with my early adopter friends and fellow marketers in one spot [segmented of course]. But the real "fun" comes from my less techy, but more exciting friends. They'll start to trickle over but if it's not fun for them immediately, they're gone and in turn their friends won't stick either.
On the plus side [no pun intended], there's no guarantee that Plus is intended to "beat" facebook. Twitter is in many ways a different service, Plus could be one as well. In fact, given the way Plus presents data and who uses it now, I'd say that it's more competitive with Twitter than Facebook at present.
The question for me is where Google takes this thing in the next 2-3 weeks. Do they go back to innovation or do they lift up the gates entirely. And if they lift them, who do they take aim at bringing over?