An internal document from Facebook says that the company has no systematic control over what purposes each kind of data gets used for. "We can’t confidently make controlled policy changes or external commitments such as 'we will not use X data for Y purpose.' And yet, this is exactly what regulators expect us to do."

This is because much of Facebook's important data is derived correlations and instances of those correlations. Those didn't directly come from anywhere, but rather was calculated based on various kinds of data from various sources, including various people's personal data.

If they find that listening to a certain song has a 75% correlation with being gay, the fact that person X listened to that song is certainly personal data. What about the conclusion that X is likely to be gay? What do various privacy laws require about data like that?

If Europe is firm about regulating the whole mass of data that companies like Facebook deduce, it might make surveillance-based advertising totally unfeasible. That would be great. But Big Data will lobby for a weaker rule, or a weaker interpretation, that would make less difference.