Google+ adoption (the sign up process) is tied to conversion from existing google services to google+ profiles and growth (in terms of increased use of the service by the users that adopt it by signing up) is tied to scale and network creation (not conversion).
They are gaining viral adoption now but are going to hit their super viral mode (when more people start spending longer periods of time on the service itself) when the people who are starting a "social network home" there start interacting there because ALL their network (or most of them) will be "born" there.
Social networks are so sticky because the people we interact with in the real world are on them, google+ has the problem of needing to bring you AND your network over in order to get you to spend appreciable time there. This is why their strategy of just tying together existing resources (where people use their products world wide) and then providing the social platform for those people to build their network from ground up there is so smart. It is a time based strategy that appeals to all those *billions* of people who have yet to be on any social network. Facebook is big yes...and all over the world yes...but Facebook connects 800 million...but there are 7 billion on the planet...that is a lot of social networks waiting to be built and a much larger chunk of those 7 billion are now using at least one google product....and thus the hook is in the jaw. If their first taste of a social network is via an automatic Google+ sign up via gmail or docs or search...then that is where they will build their "home". Facebook lacking all those distributed features has to go the other way...country by country...hopefully virally building inward....Google+ is kind of converting existing service users to social networkers in the opposite way and that is going to take more time.
I won't probably ever be a big user of google+ (outside of business and networking purposes) because most of my *social network* is HERE and not there and I have nearly 1,000 in that network....to much work for me to move there and next to impossible to get them all to move there as well....but people with much smaller networks, will find that task way easier.
Finally, by getting the deeper demographic information from a sign up that google now has via Google+ they can target their ads better world wide and charge more efficiently for them even without the growth (as defined above)...and thus make more ad money *weather the people use their G+ service or not* today.
It's a total, win win situation, win now from the sign up and conversion efficiency of the service AND win later as the loose fish all over the world start and build networks on the first social network they see (google+).