While Jay Leno has received the lion’s share of bad press for his mediocre run in prime time (4.8 million…and dropping), little attention has been paid lately to Conan O’Brien’s tenure as the fifth host of The Tonight Show. That’s about to change: O’Brien just ushered in the franchise’s worst November-sweeps performance since 1993. Already hampered by a weak lead-in from Leno, Tonight — long one of NBC’s biggest moneymakers — posted a staggering 52 percent loss among viewers (2.35 million, down from 4.8 million) versus the same month last year. (One bright spot: Tonight‘s median viewer age dropped almost 10 years, from 55.8 to 46.4.) So who benefited from the exodus? O’Brien’s time-slot neighbor David Letterman, who posted his first sweeps win in 15 years with a huge margin of victory over NBC.