@richard_millington that was a great question that got me down a rabbit hole to an interesting research article circa 2000:
This is just a slice with some interesting stats. Wonder how it would be different today
Tens of millions of people have joined a variety of Web communities (Parent Soup, Women’s Wire, Third Age, and Garden.com, among others). Their reasons for joining are both professional and social. According to a Business Week/Harris Poll in 1997, 42% of those involved in an online community said it was related to their profession, while 35% said their community was a social group, and 18% said they joined because of a hobby.
This part also has some sites you can look at. Obviously some (most) are now defunct:
The U.S. section of the Pentax site has a very active community of photography enthusiasts from around the world who exchange advice on buying special lenses, determining the best magazine to buy, fixing troublesome features, photographing butterflies, and so forth (www.pentax.com/discussion.html).
Other large consumer companies have built popular communities around associated interests. On its family.com site, Disney operates one of the liveliest bulletin boards targeted at mothers. Its discussion topics deal with parenting, marriage, health, food, education, holidays, and many other issues. Canada’s Molson beer attracts ice-hockey enthusiasts to its site with information about the sport and message boards so fans can exchange views and gossip about their teams and ice-hockey heroes (www.molson.com.) CNN hosts many discussion boards and chat sites, generated in part from viewers of its news broadcasts and in part from its regular audience that seeks out CNN when traveling. Not surprisingly, CNN has an active community of business travelers who share tips about packing, rate worldwide restaurants, and debate the hand-luggage policies of various airlines (community.cnn.com). The key is finding a related issue that captures people’s attention. People must care about the issue, have opinions about it, and be enthusiastic enough to share their views.
Advantages may well accrue to the first movers in these cases. After all, there are only so many parenting or hobby-related sites that any one consumer will belong to. Johnson & Johnson is about to launch Mothers’ Circle, a discussion group for mothers to exchange ideas, suggestions, and advice on its Your Baby site (www.yourbaby.com). Johnson & Johnson will be in competition for the attention of mothers with the many well-established, fully developed, and similarly focused communities to be found, such as iParenting.com (which has more than 200 active discussion boards), parents.com, parentsoup.com, or parenttime.com. Pets Unleashed, which attracts pet owners to the Heinz pet foods site (www.heinzpet.com), is soon to launch an Owners’ Corner where pet owners can contact each other. Meanwhile, its rival Purina has partnered up with the busy and well-established Pet Place at the iVillage community site (www.ivillage.com/pets).
I am thinking you may want to look at Usenet communities too, like I think that is how the IMDB community started.
Hope that helps