Two major members of the literati have gotten tangled up in the Net, with exhilarating consequences. Novelist John Updike made his cyberdebut this summer, contributing the first paragraph of a mystery story as an undisguised promo scheme for the bookselling website Amazon.com. In the first week after ”Murder Makes the Magazine” went up, some 80,000 entries flooded in — and were judged by the six bleary-eyed editors assigned to pick a paragraph a day to keep the story rolling (Updike will furnish the concluding paragraph on Sept. 12). Meanwhile, novelist Kurt Vonnegut found himself involved in a cyberhoax, as a global E-mail published the text of a commencement speech he purportedly gave at MIT. The whimsical essay, beginning ”Ladies and gentlemen of the class of 1997: Wear sunscreen,” was actually written by Chicago Tribune columnist Mary Schmich. Stranger than fiction, indeed.