That comes after a New York Times story this weekend suggesting that Zuck & Co. have been assembling hardware and software engineers to take another crack at developing a Facebook smartphone. That news in turn followed news that Facebook was struggling to generate revenue from mobile users, first in a revised S-1 filing in the days before Facebook's IPO, then—to much controversy—in Morgan Stanley research reports.

(Who knows what may be afoot? Research In Motion shares were halted in after-market trading pending an announcement—which turned out to be that the BlackBerry-maker had hired JPMorgan and RBC Capital Markets to advise on strategic options.)

If today's tumbling share price is an indication, a quick stab at tackling mobile is unlikely to change investors' minds. Short-term concerns over the revenue model, not to mention expected insider sales in mid-August when the 90-day lock-up ends, still loom.