Well if they were subpoenaed they would have to turn the information over.... dont ya think? Of course to get a subpoena they would have to prove it was your account first, if the cops can get a subpoena you've already opened a bigger can of worms somewhere.
If you're worried. Sign up with an email address and only use it once. Don't post pictures of incriminating evidence, or things that could be used circumstantially.... like a picture that shows some obvious connection to you. Your home, car, pets, yard..... anything that could possibly make it seem it was you posting.