Which social media sites works for you the most in case of hooking up new members?
I think twitter is the most effective one on this case.
Facebook will be the best medium due to its billions of users worldwide.
For getting new members I think Facebook and twitter both are great in their own ways. But I prefer facebook groups specially as they very easy.
Facebook, twitter, google+ are some of the social sites which are helpful in getting traffic to your site.
Both are effective in gathering a lot of users to your website. Both has a direct effect, its a matter of how your post will persuade them.