This is an established practice. For instance, in the TOS for Facebook, the following is stated (I’ve highlighted relevant portions):
User Content Posted on the Site
You are solely responsible for the photos, profiles (including your name, image, and likeness), messages, notes, text, information, music, video, advertisements, listings, and other content that you upload, publish or display (hereinafter, “post”) on or through the Service or the Site, or transmit to or share with other users (collectively the “User Content”). You may not post, transmit, or share User Content on the Site or Service that you did not create or that you do not have permission to post. You understand and agree that the Company may, but is not obligated to, review the Site and may delete or remove (without notice) any Site Content or User Content in its sole discretion, for any reason or no reason, including User Content that in the sole judgment of the Company violates this Agreement or the Facebook Code of Conduct, or which might be offensive, illegal, or that might violate the rights, harm, or threaten the safety of users or others. You are solely responsible at your sole cost and expense for creating backup copies and replacing any User Content you post or store on the Site or provide to the Company.
When you post User Content to the Site, you authorize and direct us to make such copies thereof as we deem necessary in order to facilitate the posting and storage of the User Content on the Site. By posting User Content to any part of the Site, you automatically grant, and you represent and warrant that you have the right to grant, to the Company an irrevocable, perpetual, non-exclusive, transferable, fully paid, worldwide license (with the right to sublicense) to use, copy, publicly perform, publicly display, reformat, translate, excerpt (in whole or in part) and distribute such User Content for any purpose, commercial, advertising, or otherwise, on or in connection with the Site or the promotion thereof, to prepare derivative works of, or incorporate into other works, such User Content, and to grant and authorize sublicenses of the foregoing. You may remove your User Content from the Site at any time. If you choose to remove your User Content, the license granted above will automatically expire, however you acknowledge that the Company may retain archived copies of your User Content. Facebook does not assert any ownership over your User Content; rather, as between us and you, subject to the rights granted to us in these Terms, you retain full ownership of all of your User Content and any intellectual property rights or other proprietary rights associated with your User Content.