Rather than mad, perhaps the investors are taking it as a signal that Facebook management, by buying other companies in mostly stock deals, believes that Facebook stock is overvalued.

That could be a rational rather than irrational explanation.

I think you can attribute at least some of the decline to this. Only $400mm came from cash with additional shares possibly being issued based on performance. While this alone may not be a good reason for a long investor to drop shares, if it signals something about Zuckerberg's acquisition strategy going forward, then some investors may be expecting future dilution from new acquisitions. Furthermore, structure of share classes within FB also makes Zuckerberg's whims about this sort of strategy more influential to FB's direction than it would in another company.

Rational investors don't sell shares punitively, although if they believe that they'll continue to be diluted at a rapid pace, that could account for some of the downward valuation.