What is the legality of a website that allows people to finger out people who beat their significant others?

Well you are more or less protected to post almost anything you want. In general these public forum websites can only be sued if the hostmaster of the site knowingly posts false info on their site to discredit someone. For the owner of the site anything they post themselves is subject to the same laws as any journalist, but when you have a public forum site comments and posts from people who are not the owner of the site are protected from the owner beint sued. Make a long story short who ever writes anything on the site will be sued. So if someone wants to sue another for libel it would be the writer of the post not the owner of the website.