A glittery cast featuring Matthew Perry and Bradley Whitford. An acclaimed creator in Aaron Sorkin. A goodly amount of media drool. A debut audience of 13.4 million viewers. Yes, Studio 60 had all the signs of becoming a Big Fall Hit. But after episode 1, the NBC drama about a late-night sketch show took a big fall, and the Oct. 23 episode drew only 7.7 million viewers. (On Oct. 30, the network tried fellow struggling drama Friday Night Lights in Studio 60's slot; it garnered 8.3 million viewers.) NBC denies the cancellation rumors, and notes that the show attracts an advertiser-friendly upscale audience. (The network also just ordered three additional scripts.) ''We are very hopeful that we will grow the loyal core audience that the show has already established,'' says NBC Entertainment president Kevin Reilly, through a spokesperson. But as Sorkin is finding out, it's a little harder to grow a real audience than it is to write yourself one.