Not sure but a contact saying "this service is now owned by $company, do you wish to allow your data to be retained". Similar offers could be required whenever contractual terms are altered, the default being to continue service.
That would mean that if a malevolent owner decided to change T&C and sell all your private data youd have a legal recourse with which to stop them.
Personally I think that Opera's browser based server offers a way forward whereby a user would have all their data local and a FB like service would operate as a hub/link - like how bt services are pointers to distributed data.
Surely it's already too late. They bought your data.
Why do we allow companies to sell personal data on as part of the company? Should this be legislated against?