One has to understand what "Public Domain" means. Much of what is considered "Public Domain" (PD) is anything 1924 and earlier (US, some countries 1944, 1952). That, in general, means anything created after that time is in copyright. Even those viral youtubes.
Reality is most of those folks will never pursue as they never had a commercial intent when posted. And that is what google and viacom and all those others are depending on.
[added] the creators of the content that went viral... usually from their phone or vidcam... had no commercial intent OR realization that they might have made a buck if they had only known. :)