Hi S.,
I'm not sure how much the laws have caught up with this type of activity yet, but as you research this, here are a couple of suggestions.
1. Print/save everything - postings, and website policies, including the user agreement when signing up for an account, or the site's policies on postings, membership, fraud, and privacy policy. Remember, policies can change, and content can be added/changed/deleted at any time, so make sure to print out evidence of it while it exists.
2. Contact the sites to let them know that someone has fraudulently set up an account under your name.
It will require a subpoena to the site to find the identity of the poster.
I'd recommend starting with the police to see what they can do and if they have any guidance about the laws. If they are not too helpful, don't get discouraged, just keep asking questions about the law and where you can go for help with this. The reason I recommend starting there is because attorneys are so expensive, and better to start with police if they can help, and perhaps give you some free information.
Take care,
M.