Some variations can be: "There is a hacker that will block all your friends, leave nasty comments on other's profiles, will deactivate your account, will get you in trouble with the staff, etc." another could be "Don't watch *insert deviant's name here* because they will give you a virus if you do" there are a lot more variations, but you get the idea. These journals always end with something like "If you read this warning, spread the word and share it with your friends so they won't be the hacker's next victim!"
I know, reading something like this can make you feel terrified and you might feel compelled to spread the word, you don't want you friends to be victims of this terrible hacker!
People re-post these journals with good intentions, trying to warn others about this alleged hac