Being an obsessive entertainment fan used to be so simple -- and solitary. Let's say your life revolved around I Dream of Jeannie, Elvis Presley, or even Planet of the Apes. All you had to do was go to a nostalgia convention, pony up the dough for some trinkets, and presto! your bedroom would be transformed into a shrine to Barbara Eden, the King, or Charlton Heston. The best part was that if you were shy about your fetish, it was safe from the prying, judgmental eyes of those who'd never understand. But thanks to the Internet, fandom is turning into exhibitionism. Closet devotees are learning that no matter how twisted or obscure the object of their desire, now they can communicate instantaneously with like-minded strangers all over the world.

Of course, introverted fanatics can stay off-line. But those who do venture into cyberspace to let their freak flags fly can share their personal encounters, bizarro memorabilia collections, and knowledge of arcana, with approximately 20 million others. In fact, occasionally a fan cult on the Net has become so powerful (like that of My So-Called Life) it's made TV networks think twice about canceling a ratings-poor show. (Ultimately, Life did get the heave-ho, but not before much reconsideration by ABC execs.)

The on-line migration has spawned hundreds of unauthorized, homegrown fan pages on the World Wide Web, where compulsive stargazers are loading up the Net with sites jam-packed with color photos, trivia, bios, and elegiac verse. Why the boom? For starters, it's easy. Several commercial on-line services (including America Online and Prodigy) and Net providers allow subscribers to create their own websites for free. Even on those that charge, all it takes is $100 or so and very limited technical know-how to create a relatively spartan site.

Norwegian Anders Herman Torp, a 27-year-old Woody Allen buff working on his Ph.D. in computer science, runs a home page on the filmmaker. ''I wanted to present something useful to the rest of the world,'' he says. ''I am a Woody fan, and I didn't find any site dedicated to him. So I gathered some information and pictures of him and made a page.'' Since Torp has an unlimited student Net account at the Norwegian Institute of Technology in Trondheim, setting up the page cost him nothing. He says he spends about two hours a week updating it, adding audio clips of movie dialogue and info on Allen's upcoming projects he gets from articles and E-mailed tips from fellow Woody fans.

Not surprisingly, most of the fan cults that existed before cyberspace (B.C.) are on-line too: Godzilla, Monty Python, and, of course, Star Trek -- whose subgroups have multiplied like Tribbles in the past year (there's even a Klingon Language Institute website). But, sorry Trekkies, the creme de la creme of the old-school fan sites has to be THE UNOFFICIAL ELVIS HOME PAGE (http://sunsite.unc.edu/elvis/ elvishom.html). This on-line King's palace is as cozy and welcoming as the scratch and hiss of a worn 45 of ''Blue Christmas.'' Sucking you in at the top of the page are two photos of Presley himself: on the right, the young singer in all of his greaser glory; on the left, the Vegas-period, muttonchopped royal. Below, there's his will and humorous tales of Elvis sightings on the Net. There's even a detailed account of site operator Andrea Helene Berman's failed battles with Elvis Presley Enterprises. ''[A cease-and-desist]letter told me that if I didn't remove my tour of CyberGraceland, they would take legal action against me,'' says the 24-year-old aerospace engineer. ''I am not a psycho Elvis fan who thinks he's camping out in my backyard.''