MySpace punditry

Last night I was talking to my cousin via myspace and since it was getting late I decided it was time to sign out and go to bed. To end our conversation I wrote her, "Goodnight, I'm going to get off now and afterward I'll sign off myspace and go to bed." I was trying to be clever with my dirty little pun, but since it is a phrase and not a single word does it count as homonymy? Why are puns funny? All we are doing is tricking each other when we use puns of this nature and tricking people is usually a rude thing to do. We set up our sentence so that the listener expects one thing and then  we turn it around on them. Knock knock jokes are puns and everybody knows and loves (secretly) at least one knock knock joke. They're terribly annoying but they can be really funny. Why are knock knock jokes the socially acceptable malapropisms while others make the speaker seem uneducated ?

"Orange you glad I didn't say banana again?" instead of "Aren't you glad..." = acceptable
"My colleague is a vast suppository of information" instead of "repository of information..." = bad at recommendations