As for policing, I think that would probably be where the money would be made - reputation/quality of the messages being posted.
It would end up full of fake job spam, and the listeners might want quality filters. I can imagine that you might pay to validate your identity as not-a-spammer.
I'm not saying it was a slam dunk, but a lot of talk about this back in 2007/2008 and there was a palpable excitement about Twitter apps. Within a few years the media-company mindset had taken grasp, the valuations and expectations exploded, and now we have boring old Twitter which is still a damn cool thing in its own right, but which is seen as a failure because of a combination of pedestrian vision and mismanaged expectations.