Sometimes people pass on gossip. But more often people just don't have a clue who can "see" a post and who can't, and they may well mention a post to a friend (a real friend, not a LiveJournal "friend") but only realise when they meet with blank incomprehension that that friend has been blocked from seeing the post in question.
It's interesting what gets around, though. It's interesting how complete a picture I can piece together of what's been said that I can't see, without even trying. I probably wouldn't bother trying; the content doesn't really interest me. But the information flow fascinates me; the unspoken assumptions (usually wrong) about who won't talk to whom. (I'd love to try to track the progression of a piece of news or information: to mention it in one place only and see how far it gets. The only problem would be how to determine where the information had got to: if you ask people "Did you know [whatever]?" you risk triggering false recognition.)
Personally I tend to take the view that if I say something anywhere, at all, it will escape eventually. Information wants to be free. If it's so horrible that I couldn't bear somebody to read it, then I probably shouldn't be writing it; if I write it, then as far as I'm concerned I have to be prepared to take the consequences of people seeing it.
Basically I don't like trying to solve social problems with technical solutions; if I wanted to try to control what other people said to each other, LiveJournal certainly wouldn't be the tool I'd use. I can't imagine wanting to do that, but some people are strange, eh?