Its opt-in when you sign up for a service like Gmail.
Its opt-in when you use a service like Google Search.
Its opt-in when you freely post something to Facebook.
I agree entirely, but your post hardly serves as a counter to the point I was making. I guess you were making an implication, the point of which was the same as the point you initially made, so I'll try again:
It's opt-in when you let a friend in on something private and personal.
It's not opt-in when that 'friend' lets a lot of other people know about it without your permission.
In the context of Google/FB/a.n.other corporation, the expectation/implication of privacy stems from a number of factors, including a security mechanism which is supposed to ensure that you alone can access your account (or the provider of the account can if they absolutely have to in order to continue providing the service).