YouTube, Facebook and other similar sites post popular video clips--is this legal?
It depends on the video clip. YouTube and Facebook allow the posting of video clips that may or may not be in violation of copyright. Copyright owners are expected to notify these sites of any infringing uses. Users who post clips in violation of copyright could be liable for copyright infringement.
YouTube has a three strike rule relating to copyright. If you infringe three times, your account and all uploaded videos on your account will be deleted. To avoid this happening, avoid uploading videos that you don’t own!
To learn more, visit http://www.youtube.com/t/copyright_center