It doesn't matter if a professor at Kellogg thinks it's worth 25 billion, as long as enough investors think it's worth 100 billion (as is the case today).
It is a different matter, though, if a large fraction of those who think it is worth 100 billion is just Morgan Stanley. Then there is definitely a problem.
as long as enough investors think it's worth 100 billion. . .
. . . for now, and continue to increase their estimation of the company's worth in the future.
That's a key caveat. The easiest way to get hurt by an overvaluation is to conflate what people are valuing something at right now with what it's actually worth. Overvaluations and undervaluations both tend to regress back toward a fair valuation over the long haul. Even poor valuations that most everyone agrees with for the moment. People's opinions can and will change.
“I can’t justify it,” said Anup Srivastava, an assistant professor at the Kellogg School of Management at Northwestern University. “Facebook is a great business, but it’s worth around $25 billion.”
It is a different matter, though, if a large fraction of those who think it is worth 100 billion is just Morgan Stanley. Then there is definitely a problem.