This may be proposed business model for the three previously mentioned ideas:
I am thinking of the classic idea that the basic service is free but more advanced services are not. Obviously the free msut be reasonably useful.
Posting a message on an area with an certain radio (more than X and less than Y) and with a certain duration is free.
If you want different geographical range (wider or more specific) or a different duration, you have to pay. This approach can also be used to dissuade spammers (as they usually want the maximum range and the longest duration).
An extra tuning to this business model may be adapting the free radio range and the free duration range to different kinds of content. But this may be difficult
Is it too naive? Anyway, if this business model does not work, you will always have Adsense.