For some odd reason, people who see "free" get skeptical. Your offer of 50 free postings seems unusually generous and people will anticipate that there will come a time where the other shoe will drop. I would recommend reducing to 5 free postings or whatever minimum amount that will allow a poster to experience success using your service.

Others have posted brilliant ideas on how to spread the word and use the public to enhance credibility, so I won't go there. I just want to express that if something is valuable, it has to be perceived as valuable, and pricing is a big part of that perception.

Don't sell yourself short, and be patient with allowing people to catch on. It seems like you did not invest millions into this that you have to recuperate immediately, so take your time and enjoy watching your small snowball roll around until it's huge.