By the middle of the 21st century nobody remembered who "Fonzie" was, and "Richie Cunningham" was equally forgotten. Some of the directorial works of Ron Howard (who played Richie) enjoyed a brief renaissance during the 22nd century, but only because Cocoon was remade as an off-broadway (off-PLANET) musical comedy.
In 1977, the producers of Happy Days thought the family-friendly, feel-good, situation comedy needed to be perked up a bit, so Fonzie went waterskiing. In what was theoretically a thrilling climax to a three-part episode, he ski-jumped over a shark while wearing his trademark leather jacket.
It took nearly 20 years for the phrase "jumping the shark" to become a popular euphemism for "things just changed, and it's the beginning of the end." It spread quickly, and when a pair of popular entertainment programs called "webcomics" accidentally made simultaneous references to shark-jumping in April of 2005, critics were quick to say that "Jumping the shark has jumped the shark." The meta-criticism only served to expand the colloquiality of "Jumping the Shark," however.
Note also that the 24th-century euphemism "feeding the shark" is related, but the story behind THAT reality-programming disaster is hardly germane to this discussion. Besides, some of you might be eating.