Google+ really is a Facebook killer. Here are few simple reasons why.
Google+ integrated into Chrome, which is quickly becoming the browser of choice. Social notifications straight to your browser without any extensions... Default Google+ functionality like share/+1/etc. Facebook has no chance of competing with this.
Google+ integrated into Android, the leading smart phone OS. What happens when all those hundreds of millions of Android phones start shipping with default Google+ functionality? What happens if Google decides Facebook doesn't really have a place on Android anymore?
Google+ is Google's Grand Unification. If they really planned for everything to work out like this, then they're absolute geniuses and they pulled a fast one on almost everyone. They've built the most impressive social network I've seen, completely in reverse, one piece (site) at a time.
I honestly won't be surprised if Microsoft/Facebook/Twitter start crying foul and throwing around the monopoly word. And perhaps that's why all these puzzle pieces are starting to slowly come together, because if they launched all this functionality at once, Google would probably be neck deep in lawsuits.
As a corollary to all this, if Google+ fails, Google will take a huge hit. It's a remarkable gamble that has the potential to change the web entirely, for better or worse.
EDIT: I forgot Google Maps with Google+ integration/check ins. It's daunting to think of all the ways Google can take over the world.