Google is not interested in pulling you and your massive network over to their service primarily. They want to allow people who are using their distinct services to create a social networking home on G+.
The point is often made that many people who go to Google+ from Facebook come back. I've read many formerly very active users here with big networks go and stay there. They have much more to lose by switching but went anyway...yet still they aren't the fish Google are trying to catch. Those fish are mostly not even American's for the most part they are people in foreign countries using mobile devices for accessing google services...where FB penetration is still low (the numbers of people fitting bill hover in the billions) so there is a lot more potential market there than here.
The task of building a compelling service that enables social interaction among google service users that people like is done already, now it's a war of attrition in the works. As a result, I predict Google+ will continue a moderate growth in adoption over the next couple years and will be where Facebook is with total numbers (700 million) in about 5 years. Time will tell if this prediction is true or not.
Some people think that the main prize is going after the internet's "identity system" and though this is a valuable prize. There isn't going to be a *single* global internet identity system, as it stands google has what Facebook doesn't; a suite of web based tools used by a lot more people collectively than are on Facebook (including many who are on Facebook) and they are adding social to that. Facebook has the much harder task of going in the other direction ..of tacking tools onto their social component. I definitely find it convenient to login using Facebook but I have a big network here already, I centralized here already...that's not the case for *most of the planet* they are the low hanging fruit to google and as soon as they can put up similar "login using google" buttons all over the place people who use their services will surely use them. (I actually trust google over facebook by far...as far as I can trust any public corporation) This is why the attrition that will happen will mostly benefit google over facebook over time...IF Facebook doesn't quicky gain parity tool wise with Google...that is a tall order as much of what Google has built in the ad space will be energized significantly by the social elements they've already added in this *beta version with invitation only access* of their vision of social.
Google+ growth by comparison: