The thing that most turns me off from LinkedIn is that it leaks your browsing information to the users you are browsing, through their "who viewed my profile" feature. If I want to just brush up on a contact's background before a meeting without announcing to him that I'm stalking him, I need to log out and re-browse. Major pain in the butt.
It's pretty easy to discover--under privacy in your profile. But, yes, the fact that a number of these types of settings are opt-out rather than opt-in is a pretty dark pattern.
