Why has Facebook's stock price fallen sharply?  As I understand it,  Facebook is now offering its users the right to "opt out" and not share their data.  You do not have to be as sharp as Professor James Heckman to anticipate that this will cause a selection bias issue.   Who will opt in to continue to provide their data to Facebook? How representative will this subsample be of the entire population? If advertisers are aware that the quality of the Facebook data has declined because of this selection bias issue, then they will bid less aggressively for advertising on this platform. This will lower Facebook's revenue.

Will good econometricians figure out how to do a valid Heckman 2-step correction to make kosher inferences from the data Facebook now collects moving forward? This hinges on finding Z variables that predict the probability of opting into Facebook's data sharing program but are uncorrelated with actual search behavior and "likes".   I can't think of one.  Will Facebook randomly assign different payments to people for sharing their data? This is what Frank Wolak and I did in this 2013 field experiment.

I recognize that this theory cannot explain the timing of why FB's stock price fell so sharply last week but this pursuit of privacy (and people's differential demand for privacy) poses key issues for this platform's reliability in yielding insights about the public.

I would also note that Linkedin has not been affected by these issues.   I have asked economists whether they are surprised that Linkedin and Facebook aren't integrated into "one life experience".  When we teach Edgeworth Box economies, we celebrate the integration of economies but these two giant companies thrive separately. The answer is obvious.  Facebook is about your private social life while LInkedin is about the workplace. People want a sharp divide between the two.  I doubt that political issues arise on Linkedin because people people in the for profit space know that such stirring of the pot won't help their career concerns.  I doubt that the Russians bought any Linkedin ads in 2016.