This reasoning seems flawed to me: He seems to not understand that the rational choice for individuals acting independently is often not rational for the group as a whole. The Prisoner's dilemma, Bank runs etc. are all examples of how this can happen.
If everyone else is in danger of selling and pushing the price down, you're better off selling now before that happens... and of course, everyone else would reason the same way, making it a self fulfilling prophecy.
If the shareholders had some way to collaborate and agree collectively to not sell now, his statement might be correct - but I doubt this is even legal even if it was feasible.
Of course, if you're in it for the long haul, then other people's opinions of the stock price shouldn't matter: you just buy/sell facebook stock based on whether you think the future expected dividends justify the investment.