Since The Graduate is a comedy-drama about a crazy affair and its unexpected consequences, it's managed to become popular without garnering the kind of extreme fans that Star Wars and Star Trek have. It's clearly not that kind of movie. But it's got its fans. It was especially popular with the '60s college crowd—people would watch the movie in theaters and yell "Plastics!" at the screen at the same time that Mr. McGuire was saying the famous line. Supposedly, student radicals at Columbia took time off from occupying the University President's office to go to repeat viewings of The Graduate on the sly. (Source)
(We always suspected that those student "strikes" were just an excuse to have fun.)
The film still has some fan sites scattered around the internet—like "The Graduate: A Resource for Film Students and Fans," which collects reviews and essays on the movie, along with pictures, fun facts, and other information. It's been re-reviewed on its 30th and 40th anniversary. By now, most of the hardcore fans are collecting Social Security. But they're secret subversives at heart and remember what it was like back then when everyone had expectations of you that weren't your own, and you felt alienated, angry about the war, questioning, and aimless. Benjamin was their cultural icon.