ABC's Nightline, which once faced the threat of extinction if ABC made a deal to hire Jay Leno after he left the Tonight show, now finds itself not only beating David Letterman and Conan O'Brien in the ratings but on some nights even topping Leno's primetime audience. (On Monday night, Leno drew only a 1.3 rating, appreciably below the 1.5 rating that the network has said it needs in order just to break even on the show.) Daily Variety observed today (Tuesday) that during the November sweeps, Nightline not only drew bigger overall ratings than its competitors but for the first time also edged past them among viewers 18-49 as well. Meanwhile, ratings for Tonight have plummeted, the trade paper observed, with its overall audience now down by half and its 18-49 viewers reduced by 25 percent.