LONDON — Grace Kelly was a film star before she became a princess. Princess Diana was royalty before she became a star. Meghan Markle is neither a princess nor a star, even though she married a prince and is very famous. In the age of social media, there can be no real princesses or stars, because there is no privacy and no mystique. There is no content, only surfaces, and no substance, only promises. The winners in such a media environment are unprincipled counterfeiters: actors, for instance, or politicians. Meghan Markle was never much of an actress. She is not a politician in the traditional sense, either. She has never run for office or gone through the antique procedurals of voter drives and public debates. Nevertheless, Markle, who promoted herself as a “philanthropist and activist” while she was a full-time actress, is already a politician in the emerging sense. She is an entertainer by profession, and the merger of politics and entertainment, formally announced in the 1960 televised debates between Nixon and JFK, was completed long ago. Barack Obama’s digital campaign in 2008 saw this coupling evolve into a new format, social media. The medium being the message, social media has already restructured our economic and social lives. But it is only beginning to restructure our political system and ideas. A lot of interests are vested in the current system, even though it no longer...