These services are great as long as it's part of a small, insular community of like-minded people. Once the criminal element sees it for the opportunity it is, it's over.
There's an obvious analogy here to email and spammers.
I suppose it could still work if the vetting process is better, but there will always be those who mask their identity and "sneak in". The cost of this to a criminal is probably minimal while the potential rewards are great. Perhaps the key is to create smaller "circles" of trust within the wider community.