A twatcher is someone who just reads other peoples twitter feeds and doesn't contribute anything of their own to the conversation--a twitter voyeur. A twatcher may or may not have a twitter account of their own.
Sarah: Did you see that textsfromlastnight Kyle tweeted last night. Haha, two guys.
Becky: You're not on twitter, how did you see that?
Sarah: Umm, it's called an RSS feed.
Becky: Girl, you are such a twatcher!
Sarah: I don't like using at signs and you know that!
Someone who is on twitter following a whole lot of people. Never tweets anything but watching watever body else is saying cause they dont have no life.
If you twatchin, this tweet is for you twatcher!