Surprise Hits: Facebook sneaks up behind MySpace -- and sprints past

Although it seems obvious in hindsight, the widespread adoption of social networking -- and the supremacy of Facebook in particular -- were far from guaranteed. It's not every day that a product of the ivory tower connects so easily and quickly with the public at large. But once in a while, lightning strikes.

Started in 2004 by Mark Zuckerberg, Facebook was initially open only at Harvard University (undergraduate enrollment: 6,700), where he was a sophomore. But soon after its launch, other Boston-area schools wanted in -- so Zuckerberg expanded the site, taking Facebook on its first baby steps toward today's social-network dominance (total Facebook users today: 200 million and counting).