"I said, 'Gee, how old are you?' And he said, '20,'" Fusco remembered in a recent interview. "I said, 'You think I'm going to rent you my million-dollar house?' And he said, 'Yes.'"

Over time, Fusco also came to understand her tenant's clarity of purpose. When she ran into him in downtown Palo Alto two years later, she asked Zuckerberg just how he could have turned down the reported $1 billion that Yahoo (YHOO) had offered to buy Facebook.

"He said, 'Judy, I didn't do this because of the money,' " said Fusco. "Money to him is just a vehicle to do what he wants to do."

As Facebook begins the journey to an IPO, Facebook's founder and CEO is entering a new phase: Now, he will very much have to care about the money. In the coming months, questions inevitably will be raised about how far the kid who stood on Judy Fusco's doorstep has traveled toward maturity, and whether he will be a CEO as passionate about the business of Facebook as he is on creating the addictive social software that