Simple Opt in should be the rule for all internet sites, social or not, for any information sharing of any kind. No passing info on to subsidiaries or third parties of any kind. Nothing is to be placed in the EULA or any other complicated, legal BS, which allows sharing of any information. Only the user has the option to actively selects a web page to another site if they are interested in something they see on the initial site. No negotiations, no exceptions, no back door deals. Shortest law on the books if you pass it.