ABC, which once hoped to land David Letterman and later Jay Leno to host a late-night talk show, has discovered that what it already has been airing in the time period is more potent than Letterman and might very well also wind up stronger than Leno's successor, Conan O'Brien. The website TVNewser.com pointed out Thursday that ABC's Nightline has been expanding its audience for 18 straight weeks and that its ratings during the week of May 4 were up 9 percent over the same week last year. By the same token, NBC's Tonight show with Leno was down 3 percent and CBS's Late Show with Letterman was down 2 percent. Nightline remained in second place with 3.66 million viewers, edging out Late Show with 3.60 million. The Tonight show remained in first place with 4.85 million.