tweeter

Short definition from Wiktionary

Noun

1. 1
   An electronic speaker designed to produce high-frequency sound.
2. 2
   One who or that which makes a tweeting noise.
3. 3
   One who posts messages ("tweets") on the social networking site Twitter.
4. 4
   A poster on a microblogging social networking service.
5. 5
   Alternative letter-case form of tweeter (“Twitter user”).