It's not an acronym unless it actually forms a word. For example, MADD (mothers against drunk driving) is an acronym. Laser (light amplification by stimulated emission of radiation) is an acronym that formed a new word. One could probably argue that LOL is an acronym as well.

STFU, like most internet abbreviations is not actually an acronym since it has no resemblace to any word or sound whatsoever.