Oh, sure, Microsoft, Google, and Yahoo count more users. But one can hardly claim people feel much of a relationship with those giants' Internet archipelagos. People Google what they're looking for and leave; they check stock quotes on Yahoo Finance; they read their Hotmail. None of those activities match the thrill of even a poke on Facebook.

Zuckerberg could screw it all up. The Harvard dropout is his own worst enemy. But let's be clear: That's because he is his only enemy. Facebook has lapped MySpace in race after race, and is now twice its size. Twitter is no threat to Facebook. Nor is Google's inchoate plan for a social network; as always, Larry Page and Sergey Brin's thousands of engineers are so focused on making websites useful that they are unable to make them livable.

Facebook could run out of money. Infighting might tear its executive team apart. The latest redesign, or the next one, or the next one, might turn users off and make Facebook the new Friendster. But all those fates are in Zuckerberg's hand.

You got 200 million people on Facebook, Mark — just like you imagined back in your Harvard dorm room. Now don't screw it up.