A person who is impersonating another person via Twitter.
Person A: Did you hear that Jeff was receiving tweets from his wife?
Person B: No, What did she tweet?
Person A: She was saying that the pool guy was better in bed than the gardener, but it was a Twitterpostor. His best friend created a twitter account under his wife's name and he was the one sending out those tweets
Person B: Aren't you his best friend?
Person A: Yup!