Who goes on such a site? Jobseekers. Which jobseeker has an incentive to retweet a job s/he finds interesting? Usually YOU want the job you find interesting. Why retweet it when this makes it harder for you to get the job?
Jobseekers are not expected to retweet. Company stakeholders and relevant jobseekers friends are.
And what happens when you add retweeting spambots into the mix?