Is America Online ready for prime time? Its PR department thinks so. In recent newspaper ads, AOL boldly celebrates itself as ”prime time’s hottest new network,” backing up its boast with a chart showing its audience numbers climbing past MTV’s and CNBC’s during the all-important 8-11 p.m. period. Calling the ads ”ridiculous,” MTV’s Caroline Mockridge says, ”You really can’t compare an online service that covers everything from shopping to gardening — with an audience that ranges from kids to senior citizens — with MTV, a music network for 12- to 34-year-olds.” AOL’s Wendy Goldberg disagrees: ”AOL is not a computer company; it is a media company playing in the same space as [MTV].” Even so, MTV and CNBC are still just cable channels. On any given night, millions more catch NBC, ABC, CBS, Fox, even The WB. A more revealing stat: Even if all 11 million members could log on to AOL at once, NBC would have roughly three times as many viewers with ER alone. The reality is, ”prime time’s hottest network” still has a long way to go.