We are building a niche Social Networking/Market Place website, but we have been getting complaints from users that they've been contacted by lots of scammers. Does anyone know the strategies that social networking or market place sites such as Facebook, Linkedin, Alibaba, etc... use to try and stop this type of unwanted interactions? The only ways that i've found was to have users "be friends" before profile information is shared or they can send messages to each other. Or charging a sign-up fee for users (this is possible since we are a B2B site but definitely not a preferred option).
You've pretty much mentioned the only real solution yourself. If profile information is made freely available then that's what's going to happen. Look at all the hassle regarding Facebook when they changed their default privacy settings. Users should have the choice whether the share profile info with "friends" or make it available to everyone and anyone. If they make it freely available then you get the scenario you now have. You can't police the whole thing although you must take reasonable care to keep users safe. You can react to complaints, and possibly create a more rigorous sign-up procedure (don't know what), but restricting access to private info is the only way this will stop.