You, me and The Lorax all agree its possible.
We just hold different opinions regarding under what conditions it is likely to occur. Google or Facebook's proprietary access to the data they have collected only makes strategic sense so long as there is strategic value in exclusivity. Given that there already are a variety of ordinary existing circumstances in which exclusivity is not strategic - e.g. subpoena - the argument that it is only in extraordinary business circumstances that the data might be sold needs a stronger case made in its favor.
We are not talking about a few million dollars, but several billion or more. Just look at patent portfolios for a parallel way in which the market may suddenly revalue intangible assets. Particularly consider the case of Nortel and if Facebook could experience a similar demise over the next twenty years. Even if Facebook stopped collecting data today, most of it would still shed light on living people.