So @sysomos releases a study that looks at the level of retweeting and replies that occur on Twitter.
This is not promising for those that want to push using Twitter.
However, there is one thing that helps to know from the study.
Approximately, 95% average replies and retweets happen within the first hour of publication.
The other "good" news: this does not account for click throughs. There are TONS of tools for tracking the links you post. The numbers vary but the click through rates are fairly low compared to other material like email. But if a celebrity can't get better than 2%, you will probably be happy getting 1% and most of the articles indicate that it is quality and not quantity at that point.
You really do want dedicated folks whether it is Twitter, Facebook, or you blog, instead of any old person off the street.