My favourite social media marketing PR disaster was Rentokill's where they managed to combine getting caught blatantly lying in a press release which blew up live on Twitter in conversation with the popular Ben Goldacre of Bad Science with a simultaneous intrusive and unpopular twitter following campaign, complete with patronising press releases when they got caught. Absolutely splendid.
Rentokil make dodgy claims about imaginary bugs – Bad Science
Our Recent PR – A Clarification And An Apology | deBugged
Why Is @Rentokil Following Me? | deBugged