Obviously, there's been fan cultures in the twentieth century, like Star Trek fandom or D&D fandom (in fact, many play-by-post games originated via snail mail, the most famous being probably Slobovia).

Before that, such "subcultures" were fads or intellectual currents, often started by influential novels. For instance, "The Sorrows of Young Werther" by Goethe, inspired youth to dress like the fictional Werther, write Romantic poetry and was often accused of encouraging them to commit suicide. For many, Sherlock Holmes fans were considered the first true fandom, meeting with the author, writing fan-fiction or discussing the merits of the book.

Do you mean subcultures who weren't actually fandoms? Because some fandoms pre-date common Internet availability-Star Trek is one example; http://en.m.wikipedia.org/wiki/Star_Trek_fandom (complete with hard-copy fanzines, including those of a slash-fiction nature).

Assuming you do mean groups that aren't fandoms, could you clarify what behaviours you mean please? Extreme devotion/obsession? Convention attendance? Both could apply to most religions.

"fandom" existed before the Internet - it grew out of the SF community, through zines like Ansibles and conventions. Its early roots can be traced back to the "amateur presses associations" of the early 20th century, where such luminaries as H. P. Lovecraft were first published. These people also had a habit of corresponding between them a lot (HPL's correspondance is three or four times the size of his fictional works).

Although I'm not sure there are many academic sources on the subject, this blog post by Tor editor Patrick Nielsen Hayden is a great look at early fandom by someone who's one of the beating hearts of the current SF community. I'd also recommend Jo Walton's Among Others for a beautiful look into the life of pre-Internet SF fans, complete with the drama and fads we all know and love. One particular incident I remember from the book was the revelation that author James Tiptree, Jr. was in fact a woman, causing much discussion in the letters section of fanzines and among local fan groups.