A fundamental limitation on contracts is you must gain something for any contract or contract update to be binding. So EULA's are only enforceable if you don't already have the right to use the software / service. In other words, Sony could not prevent people from using other OS's on Plantations that where not updated, but a patch could constitute value that allows them to get a new contract with you. As to a service like Facebook things get a little more tricky (and I don't know how that works out).