NEW YORK — One month in, NBC’s generational trade of Jay Leno for Jimmy Fallon at the “Tonight” show is succeeding beyond the hopes of executives who engineered it.

Fallon’s fast start is clear in television ratings and even more stark in social media metrics. While too early to declare a new king of late-night TV, the transition is a marked change from how badly NBC fumbled the short-lived switch from Leno to Conan O’Brien in 2009.

“As a guy who’s been doing this for 36 years, I don’t allow myself to think about this level of success,” said Ted Harbert, NBC broadcasting chairman. NBC had hoped for an increase in young viewers and steeled itself to lose some of Leno’s older fans, but Fallon’s reception was a surprise.

When Fallon premiered on “Tonight” during the Olympics, the franchise hit numbers unseen since Johnny Carson’s last week in 1992. Things have settled down but Fallon is still comfortably on top. During the week of March 10-14, Fallon averaged 4.26 million viewers to Jimmy Kimmel’s 2.83 million on ABC and David Letterman’s 2.78 million on CBS, the Nielsen company said. Fallon has consistently topped the 4.1 million viewers that Leno averaged this season before leaving.

Fallon’s lead over his rivals is more pronounced among viewers aged 18-to-49, the demographic NBC bases its advertising sales upon.