I wouldn't post about private plans on someone's wall. Confirming a meeting, for example, is a no-no. The exception would be if it's someone's business Facebook page and I was asking a question that other people might be able to answer. For example, if I joined my yoga instructor's Facebook page, I might ask, "Is anyone else attending the 'yoga in the trees' class today? Should I park near the north entrance or the west entrance?" Ditto for an event page.
However, I wouldn't ask about personal matters on someone's public page. If I think there's any chance that they wouldn't want others hearing about it, it's not going on their wall. I think a lot of folks are just oblivious. They don't think that someone else might see their post. It doesn't occur to them that not everyone is invited to the party, or that the owner of the wall may not have told everyone she's looking for a new job, etc.
(Edited for clarity.)