Dr Johnny Ryan:

Google and Facebook will be unable to use the personal data they hold for advertising purposes without user permission. This is an acute challenge because, contrary to what some commentators have assumed, they cannot use a “service-wide” opt-in for everything. Nor can they deny access to their services to users who refuse to opt-in to tracking.[1] Some parts of their businesses are likely to be disrupted more than others.
 The GDPR Scale
 When one uses Google or Facebook.com one willingly discloses personal data. These businesses have the right to process these data to provide their services when one asks them to. However, the application of the GDPR will prevent them from using these personal data for any further purpose unless the user permits. The GDPR applies the principle of “purpose limitation”, under which personal data must only be “collected for specified, explicit and legitimate purposes and not further processed in a manner that is incompatible with those purposes”.[2]
 Google and Facebook cannot confront their users with broad, non-specific, consent requests that cover the entire breadth of their activities. Data protection regulators across the EU have made clear what they expect: