Less than three years after emerging from nowhere, the hot social networking website MySpace is on pace to be worth a whopping $15 billion in just three more years. Or is it? And is the much smaller Facebook really worth the $900 million or more Yahoo is reported to have offered for it? The problem, say Wharton experts, is a dearth of information — including data on expected revenue generation and cost structure — to plug into the standard valuation models.