Someone who Twitters obsessively on the social networking site "Twitter." Twitter + Geek = Tweek. Often appearing to be normal people on the surface, they will quickly reveal their inner geek whenever anyone mentions Twitter in a conversation, at which point they will tell everyone within earshot about all the hilarious and uninteresting things their friends and acquaintances have recently tweeted about.
See also "Twatter", someone who tries to accumulate a large following on Twitter in order to hide the fact that they are actually a giant twat.
Rational human: "I can't believe CNN is reporting on what politicians are tweeting from the halls of the Senate...or that politicians are tweeting in the halls of the senate, for that matter."
Tweek: "OMG I LOOOOOVE TWITTER!!! My friend Jessica just tweeted about how the guy next to her in line at the DMV just farted and she said it smells like a mix of hot garbage and a yeast infection!!! How funny is that?!"
Rational Human: "I hate your face."