There are huge privacy concerns in the present day, but there's the other side of it too: today we consider it a treasure when we find "Maximus sucks a big dong" type graffiti on some wall in Pompeii. The glimpse into the life of the Romans is itself exhilarating. A thousand years hence you won't be around to care about how your embarrassing posts affect your reputation, and those who find it might be more grateful for the glimpse into early 21st century life than inclined to snigger or cringe at your comment.