Less than three years after emerging from nowhere, the hot social networking website MySpace is on pace to be worth a whopping billion in just three more years. Or is it? Is the much smaller Facebook, run by a 22-year-old, really worth the $900 million or more Yahoo is reported to have offered for it? Maybe. Or maybe this is Dot-Com Bubble, Part II , with MySpace, Facebook, YouTube and the other new Internet phenoms destined for oblivion when the fad fades. “What makes this hard is that these companies seem to be so many years away from the kind of earnings that the valuation numbers are forecasting for them,” says Andrew Metrick , finance professor at Wharton. The $15 billion MySpace figure “would imply that a lot more people will be on MySpace than are currently on it.” While the social networking sites vary considerably, each relies heavily on content provided by users who can post personal profiles and build networks among friends and others with shared interests.…