Just because one person values something at a number doesn't mean anyone else (or enough other people) do. The price of their shares tell you the "Current Market Value," but they don't tell you what it is worth in the long term.
Things are only worth what the next person is willing to pay for them, not what the last person actually paid. People buying the Groupon IPO were mostly going with the "greater fool" theory of investing. A money losing company with lots of accounting issues is not something that generally just goes up and up in value.
>> they don't tell you what it is worth in the long term
This reminds me of the Keynes quote: "The long run is a misleading guide to current affairs. In the long run we are all dead."
More on topic, the long run as an abstraction is limited in its utility for investors. This mostly because investors (like gamblers at a roulette wheel) have to place their bets while the wheel is spinning. Without taking a position on GRPN, I'd say that the "long run" verdict is hardly in at this point.
Things are only worth what the next person is willing to pay for them, not what the last person actually paid. People buying the Groupon IPO were mostly going with the "greater fool" theory of investing. A money losing company with lots of accounting issues is not something that generally just goes up and up in value.