My choices for the most significant public phenomena of last year are associated with traditional media rather than the social kind, which isn't to say these phenomena didn't impact social media and vice versa, only that TV, newspapers and magazines still affect our perception of the world.
Media topic of the year — the Asahi Shimbun: It's not unusual for a media outlet to become the subject of news reports, but national newspaper Asahi Shimbun's infamy was special. As Japan's nominally liberal daily, it has always been the target of reactionary hot-heads out to defend everything they hold dear about Japanese character and its culture, and at least one Asahi reporter was murdered in the past for just that reason.
Last summer, the paper retracted stories it had run since the late 1980s about the abduction of World War II "comfort women" in Korea based on the assertions of one person, Seiji Yoshida, which were proven false back in the '90s. Though the retraction only covered Yoshida's version of events, it was cited by historical revisionists as incontrovertible proof that the Japanese military did not force girls and women to sexually serve front-line Imperial soldiers, ignoring mountains of evidence, both documentary and testimonial, that said otherwise.