n. An angry or aggressive protest on Twitter, particularly one seeking justice or vengeance.
2012
The Internet has been good at spawning neologisms. One that is particularly indicative of the times is "twitchfork mob:" a group of people using Twitter to protest, harangue, or expose people they don’t like or agree with. In 2011, after the British High Court barred media from revealing the identity of a soccer player who had an extra-marital affair, over 70,000 people took to Twitter to name and shame the sportsman.
2012
Local and international firms must take great care to avoid using advertising that projects racial stereotypes or racism. Even if these offenses result from innocent mistakes, image problems can spread fast, especially in a world of angry Twitter mobs wielding "twitchforks" around the clock.
2009 (earliest)
Tweeple! Its time to get our twikes and twitchforks! The Times basically calls us narcissistic asses!