I have to admit I'm a little embarrassed about being on Facebook. I'm too old, too busy, and frankly just not cool enough to feel comfortable on a site clearly built for people almost twenty years my junior. But I'm glad I joined. Facebook has helped me better recognize a design pattern that seems to be extremely successful and is shared by Google, all social networking sites, and a startup you'll be hearing more about from me in the future.
The basic idea of this pattern is to manage not objects (web pages, college students, places, companies, stocks, whatever), but to provide an application that allows people to define the relationships between those objects—relationships that are otherwise tacit, implicit, and undocumented—and then create a graph of those relationships on which to hang a business.
When Mark Zuckerberg talks about Facebook's "social graph" being his company's secret sauce he's exactly right. FaceBook is so explosive because it has not one graph (like LinkedIn), but many. Facebook's social graph tracks not only explicit "friend" relationships between people but also implicit relationships based on school, company, city, and arbitrary groups. As soon as you sign in you're barraged with questions about yourself so the system can start connecting you in as many graphs as possible. The more connections, the more vectors for viral spread of, well, anything, including information, applications, and money.
Similarly, PageRank allows Google to build a huge graph of the web, which provides a fertile latticework on which to grow an unbelievable search—and by extension—advertising business. Google Maps uses coordinates on a map (graph) of physical space to relate location-based data (like where the nearest Starbucks is) to other location-based data (like where I am). It's a more familiar graph which makes it easier to navigate, but also a better-documented one—there are lots of ways for me to figure out where the nearest Starbuck's is—and so of relatively lower value. I can't imagine the depth of value in the graph Google is building by analyzing not only with whom you exchange emails but also the content of those Gmail emails.
I think too many people get distracted by the "social" aspect of networks like Facebook and LinkedIn, but at the same time I readily admit that social relationships are among the strongest and least well-documented that exist, so they're ripe for the picking. But there are others. The companies that figure out which graphs are the most valuable might just be the home runs of the current era of business.