Twitter Postings: Iterative Design : Jakob Nielsen did five rewrites on a single tweet.
I don’t have all the guidelines for stream-based postings yet, because we’re still conducting usability studies (particularly of B2B users, like my audience). But, based on the user sessions I’ve observed already, I put this posting through 5 rounds of iterative design.
He also talks about this tidbit when discussing why he keeps his tweets to 130 characters:: to allow “slack” for retweeters.
(Keeping tweets below 130 characters won’t be a long-term guideline because Twitter is redesigning to remove the source attributions from the main message content for repostings. Until this redesign goes live, however, it’s best to leave slack in your original postings if you expect followers to share them.)