Not just data volume
They also have to be a bit worried about the actual value of the datasets.
How many different users, how many changes to preferences etc, the effectiveness of identifying more sepecific preferences in targetting - eventually all these things will be profiled by the advertisers.
I can't see the value holding - it gets more expensive to be more specifically targetted and the information is less reliable. There will come a point at which the drill down is no longer worth it and then it all just goes back to trending - although in much smaller groups. There may be value in assessing individuals strength of affiliation to trend groups but once the mass data input is over FB could be in real danger of being of very little value to advertisers.