Much like the technology industry you can go vertical (and address a specific area top to bottom) and then build up your verticals, or you can go horizontal (and reach over everything, albeit quite thinly) and hope to bulk up. From what I can see, Google+ is going to be a horizontal play, attempting to drape a social layer over every Google product. That gives it a completely different market shape than services like Facebook, Twitter and Skype.
History has a number of examples of two competing giants where one is horizontal and one is vertical, and I suspect that the winning giants will be spread fairly equally over the two methods. What is clear is that vertical is quicker to explain and understand than horizontal. Google is not only in horizontal mode, but has no first mover advantage, and has a number of failed "almost horizontal" products such as Google Buzz that can trip up Google+.
I think it’s going to be a very tough road for them to have Google+ accepted and become part of the fabric of people’s life – especially as there seems to be little portability of information between Google+ and the current social networks. It’s asking a lot to trust Google and leave everything else behind, and unless you are seriously ADD, you’re not going to maintain two major social graphs.
So where’s the draw to attract people away from the verticals of Facebook and Twitter?