Since abandoning its experiment with The Jay Leno Show during primetime, NBC has remained in last place among the major networks in the 10 00 p.m. hour, but its ratings have risen by nearly half during the time period. Leno had been averaging 5.15 million. But the current NBC occupants of the 10 00 p.m. hour have been averaging 7.44 million, the Associated Press reported Wednesday. Moreover, although Leno's Tonight show audience is down from what it was prior to the late-night debacle, he has been edging out chief rival David Letterman on nearly a nightly basis. Last week Leno averaged 4.3 million viewers to Letterman's 4 million. That lead could be eroded still further if, as expected, Leno faces competition from Conan O'Brien either on Fox or on cable next fall.