Facebook’s explosive growth continues unabated. They provide over 400,000,000 people with an incredibly powerful, flexible web tool. For free. And all they get in return is the most valuable information in the world.

Known as the ’social graph’, it’s the map they’ve made of your real-world relationships: friends, colleagues, bitter enemies, people you once made eye contact with in your high school cafeteria. The social graph is who you’re connected to, how you’re connected, how often you connect with them, etc. It’s what facebook is leveraging—along with other demographic data, like your age, where you went to school—in order to sell highly targeted space to people who want to sell you stuff.

Typical social graph

So when you think of what ‘the social graph’ represents—that it’s really just a chart of your real-world relationships—it’s kind of a remarkably simple idea. Facebook is a mapping service for friendship and acquaintance; people with whom you have some real world connection, however insignificant. Pretty simple.

But when I think of my friends and Facebook acquaintances, they include a lot of people I met because of what could ultimately be described as chance circumstance. The people I went to school with, who I happened to be in class with, people who happen to live in my neighborhood, or who happen to work where I work, or share my interests, go to shows I’ve gone to—although most of us chose to be where we are, there is undeniably a high level of chance involved in meeting anyone. Except on Facebook, where—for most people, anyway—you know everyone. (In fact, the idea of someone you don’t know friending you is creepy to most people.)

Yet some of the sites that are getting a ton of media attention these days are designed to connect you to strangers: ChatRoulette, Twitter, FourSquare (in a way). Is it ennui? As our personal “social graphs” have matured is the thrill gone? Is the end in sight for Facebook?

Hell no. These services are connecting people to total strangers in more extreme, intimate ways; connecting you live via video chat in ChatRoulette, or in real life, like (I guess) with FourSquare, or in a kind of Internet-wide campfire chat, like Twitter.

They may be entertaining. They may be valuable. But they will never replace your Social Graph (they may add a person or two to it). But Facebook (lets face it, MySpace, Orkut, Buzz, etc. are fading in relevance as FB corners the market) is becoming the Internet of things you really care about within the Internet that has grown beyond anyone’s ability to comprehend. These chance connections that define our lives are being indexed and archived and processed. And, for better or worse, its value to marketers is growing exponentially.

Facebook is going to win.