Used when you want someone to shut up (you're not exactly trying to be polite,) yet it's not considered as an insult like STFU. Also it can be used when answering a stupid n00b question, or after scolding someone.
Stop that or I'll ban you. kthnxbai.
^ above isn't as insulting as STFU N00B!
2) Officially an abbrieviation of 'Ok, thank you, bye.'
3) Most commonly used sarcastically after an insult to try and make it sound jokey (it usually isn't...)
Girl: Hey, d'you fancy shutting the fuck up now? kthnxbai!