1) Occurs most frequently when comments are unflattering or incriminating. Also can be necessary to delete over posts left by the person you're casually banging, in attempt to lay claim on your or salt your game.
2) Often required after drunkbooking
2) "I can't believe I wrote 'I love you' on my exes wall and commented on like 15 pictures.... I hope I post-censored before he woke up this morning!"