For a near $100B valuation you need annual profits of up to $10B instead of their current $1B.
So from their starting position:
$1B .... lets say they double membership
$2B .... they double average spend by members
$4B .... they double it again
$8B .... still not good value.
That doesn't mean I won't buy FB shares on day one - with so much hype the stock could easily double - the market can be as crazy as it was in 1999-2000.
But long-term I wouldn't hang on to them without huge increases in profits.