@Christian Bundy, AFAIK, if there is no contract, then copyright belongs to the author. So formally everybody is infringing copyright here even if they do not distribute movies or music.

As for solution, if X wants to free their followers from responsibility, X needs to attach a license to their posts. Adding a field with a license name is technically efficient, but I do not know whether it would have legal weight.

There are already beautiful and renowned licences like Creative Commons and GFDL. I see people explicitly publishing their blogs under free licenses, so, IMHO, these licenses will definitely have their proponents. If somebody wants to protect their posts from spreading by some complex copyright schemes, they need to muster their own resources for that protection. Copyright protection is not free. I suspect that few people will actually do this.