$my_url is the postback URL. It's where Facebook will post its response when you're asking for oauth credentials. If you're using localhost you can provide that as the URI for facebook. You can even provide the port.
E.g. for a rails app http://localhost:3000 is acceptable. Since facebook is posting back to that uri via your web browser, it will obey any rules you place in your hosts file as well, so you could use the production uri locally.
if you have www.myapp.com, but then add it to your hosts file as
127.0.0.1 www.myapp.com
it should also work.
