The Copyright Office keeps a list of designated agents on its Web site. Even though every service provider is, technically, supposed to have filed such a notice with the Copyright Office, clearly many have not done so as of yet. In fact, during my quick search only about 50% of the sites I work with regularly had a designation on file.
However, many have including a great deal who don’t publish this information on their Web site. If you’re struggling to find out who you should write about an infringement claim, this is probably the place to start.
However, it’s still a lot easier than digging through a host’s terms of service to find the email address or fax number that you need. Also, it lets you know whether or not a host will require a DMCA notice before taking action, saving you time by letting you give them exactly what they want the first time.