The United States Copyright Office (CO) released a reminder that previous paper filings designating agents for the receipt of take-down notices from copyright owners will only continue to be valid until December 31, 2017 (see https://www.copyright.gov/newsnet/2017/683.html).
By that date, existing agent designations must be updated using the CO's new online registration system. New registrations of designated agents also must be made online. The DMCA Designated Agent Directory's homepage is at www.copyright.gov/dmca-directory/.
Any blogger, website or other online service provider who/that hosts content posted by third parties (e.g., User Generated Content or UGC) that might possibly infringe someone else's copyrights must electronically file an agent designation as a condition of receiving safe harbor protection against copyright infringement claims under Section 512 of the Digital Millennium Copyright Act. The current filing fee is six dollars, a pittance compared to statutory and other damages available in copyright infringement cases. Of course, registering a designated agent to receive take-down notices is just one necessary step to limit liability for materials online (e.g., see 17 USC 512).