Easy answer: because Zuckerberg did it better. If you look at the early history of Facebook, you'll see that almost nothing about it was a new idea. Even if it's true that the Winklevosses came up with a plan for a Harvard social network first, they were obviously inspired by other sites. Social-networking sites—even ones focused on college students—had existed long before Facebook. The real value of Facebook wasn't that it did something new, but that it did something old better—faster, prettier, more useful, and more addictive. This is a story we've heard before in the likes of the iPod, the iPhone, the iPad, Windows, and Google. None of these were new ideas, but we shouldn't think any less of them because of it. Ideas are overrated. In technology, what really matters is execution.
What is clear about the Facebook story is that, from the very earliest stages of the company, Zuckerberg alone understood how big his idea could be. He cannily named the company Facebook (originally Thefacebook, since facebook.com was taken), because he planned to replicate it at many different schools; the Winklevosses' Harvard Connection is comically insular. Zuckerberg made a series of smart decisions to avoid the fate of previous social networks. As David Kirkpatrick writes in The Facebook Effect—a history of the site that's a lot more honest than The Social Network or the book that inspired it—Zuckerberg and the early Facebook team were obsessed about the site's performance. Friendster had rolled out too quickly and had been rendered unusable by server problems. Facebook carefully staged its launch; at first it jumped from Harvard to just a few other selective universities, ignoring pleas from hundreds of other schools.
Zuckerberg was also wise to the limits of MySpace, which at one point looked to be growing into an unstoppable force. MySpace made all profiles public by default, and it let people design their pages in any way they wanted. These features proved very attractive to a certain class of users—teenagers—but Zuckerberg saw that it would never really catch on with everyone. Zuckerberg wanted people across all demographics to feel comfortable on Facebook, and that meant enforcing some rules—you had to use your real name, you could only see your friends' profiles, and you couldn't have your page start auto-playing "Hollaback Girl." (Yes, believe it or not, Facebook was once lauded for its simple design and strong privacy features.)
Of course, Zuckerberg and his team did come up with a few genuinely new ideas for Facebook, and these certainly played a big part in its success. In 2005, Aaron Sittig, one of the company's first user interface designers, came up with a unique option to add to the site's otherwise barebones photo-posting service—tagging. It quickly became the site's killer feature, and it cemented Zuckerberg's idea that the power behind Facebook—the reason people kept coming back to the site—was the way it kept you informed about everyone around you. To make that activity maximally addicting, in 2006 the site launched the News Feed. When Facebook turned that feature on in the summer of 2006, the very first comment it got in response was, "Turn this shit off!" Hundreds of thousands of people joined anti-News Feed groups, and student newspapers worried that the feature would contribute to stalking. Zuckerberg barely budged, and now we cannot imagine life without this stream of information. Again, what mattered wasn't the idea, but the fact that Zuckerberg had the smarts—or the gall—to stick with it despite his users' anger.
To be sure, this argument doesn't absolve Zuckerberg of any of the trickery that his rivals allege he committed during Facebook's founding. But it's a mistake to suggest that if only Zuckerberg had played more fairly, someone else would now be heading a world-changing, billion-dollar company. That's nonsense. Facebook is worth billions only because the company has doggedly outmaneuvered all rivals by building products that everyone wants to use. And it did so only because Mark Zuckerberg was there to make that happen. Social networking may not have been his idea. But there's nobody else who deserves more credit for it.