Well, then, Microsoft is more then welcome to make that claim and go to court, and if the court eventually finds that Google indeed does have such a monopoly - well, then they will be subject to antitrust rules.
Until a court says otherwise, nothing compels Google thus.
Just remember that according to the courts, Google doesn't even have a monopoly on search. It is far fetched that they have a monopoly on "community contributed videos", if that's even a market sector that would be considered for antitrust.
Hell, Google doesn't even allow a significant portion of the online video market to participate on Youtube. They are anything but choking out competition; they are handing their competition a huge feature on a silver platter.
Until a court says otherwise, nothing compels Google thus.
Just remember that according to the courts, Google doesn't even have a monopoly on search. It is far fetched that they have a monopoly on "community contributed videos", if that's even a market sector that would be considered for antitrust.