Through the results of a rather interesting survey it would seem that about 34.5% of the US population is not using Twitter or Facebook. Which probably goes against what you might have thought. It would seem like Facebook and Twitter both have just about as many users as they could possibly get. They are already pushing the boundaries for just who would use a social network. They're both turning their strategies to how to better monetize the users they have than to push to try to get more users. Which makes it all the more surprising that there is still a vast majority of the population using neither service.
I should note, this survey was not at all comprehensive. It covered quite a bit of people, but it was not a true survey of the entire US population. But even so, there do seem to be quite a few more people that can still be brought into the social network scene than previously thought.
Just something interesting to note, I suppose.