I think your non-techie friends are just assuming that the folks that they want to see what they write, will, and those they don't want to see what they write, won't. And, mostly, they're right.

The biggest problem, for me, is that who can see these things is not up to only those involved in the conversation. It hardly even matters what the exact effect is of the rules currently in place, since they could change at any time. The rules on who can see my email inbox don't change. (I think this was what really upset people about Google Buzz: suddenly the rules around who could see what changed, making everyone on gmail who cares about that sort of thing insecure about their own email.)