"The site was an instant hit: Within two weeks, thefacebook.com (as it was then called) had more than 4,000 members. Before the semester was out, the phenomenon began spreading to other universities." ~National Post, Canada.com
Facebook has definitely done something right over the years. I think the most important of those decisions was to open only to a certain crowd first, college students. Then after a few years, after those students graduated from college, they opened to the public so those same people would still be able to communicate with the friends they'd made over the years.
Now, I don't think this was done on purpose, or as a thought out plan from the beginning. It just happened that way, and it's a good thing too. What would social networking be without Facebook. Maybe nothing at all.