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.
