A while back I posted about some addlepated remarks about English usage being made in a conversation on Facebook by a friend of a friend, and was accused of violating her privacy. Yesterday, during the annual twitting of ill-informed people who refer to St. Patrick's Day as "St. Patty's Day," I was accused of stalking by one such person.
Neither party seems to have understood that posting on Facebook and Twitter is publication, and that any sense that such exchanges are private is an illusion.
Before the Internet, there were clearly understood conventions. If you wrote a letter to someone, the recipient owned the physical object, but the writer retained ownership of the text, which was not to be published without permission. Conversations were assumed to be private unless made audibly in a public place. It was thought rude to listen to other people's telephone conversations on a party line (yes, I remember that), but it was widely understood that everyone did so and you should be discreet.
Facebook is rather like the party line. What you post is seen not just by the people you have friended, but by the people they have friended. And on Twitter your tweets are accessible not just to the people you follow and the people who follow you, but to anyone who has a Twitter account, or can do a Google search.
When you post something on Facebook or Twitter, you are, in effect, contributing to a public newsletter or conducting a private conversation on a crowded bus. Your expectation of privacy is nil, and you have no business taking umbrage when people comment on your ill-considered remarks.
It is not clear that any settled online decorum has developed, or is likely to. Is it impolite to comment on what people say if you are not Facebook friends or Twitter followers; that is to say, strangers rather than acquaintances? Perhaps one could make such an argument, but the Internet has not been notable for formal courtesies.
Better to assume that whatever you say will be heard by people you don't know, perhaps wouldn't care to know, and that you will in turn likely hear from them.