Color me skeptical. I'm no anthropologist or sociologist, but I don't think the critical factor is something wired into our Monkey Brain Release 3.0. I think it has more to do with the amount of time you have available to maintain relationships, crossbred with whatever passes for social skills and the ability to remember names and faces. Some of this may be biologically determined, but a lot of it is dealt to us by our families and the societies we grow up in. I would go so far as to argue that the famously cliquish and superficially polite nature of Minnesotans derives largely from their small-town origins, which have been duplicated in the larger cities by the much-ballyhooed neighborhoods, which serve a similar social function. Still, I think it's entirely possible for someone to know and care about more than 150 people, if they travel enough often enough and mix with enough other people. I think the opposite is also true, that there are people who can barely remember the names and faces of their immediate family, much less the second cousin once removed they don't see but once every couple of decades or so.
Either way, because you only have so much time to spend wandering around awake, how many and how deep your relationships are is largely a function of how much time you spend with those people. As any priest or marriage counselor will tell you, if you're spending more time playing mah-jongg with your personal computer than talking to your wife, the odds are good that the relationship ain't gonna last. On the other hand, a friend probably won't mind so much and a casual acquaintance still less - so long as you're not doing it when you're hanging out with them instead of having a beer and watching the game. This is also why we tend to look for people who have similar interests to ours - it makes it a lot easier to pursue your hobbies if the wife or GF is also into those things.
Is social networking software going to change that? I really doubt it. It may add more acquaintances to your address book, it may increase your chances of finding That Certain Person with whom you want to defy the divorce statistics. Because, after all, statistics describe average people, and we are so not average, right? ;)