Years ago before any of the major social networks popped up on the internet every little niche had their forums on the web. Some of them had tens of thousands of active users talking about anything you can think of. From restoring cars to training your dogs.
All you had to do was perform a search in the reigning search engines of the day, and you could find a great forum to participate in and started building relationships with the people on the forums.
Nowadays, Facebook Groups have pretty much replaced those niche forums. This is great from the user point of view because you can participate in 30 groups at the same time and it all runs through your Facebook newsfeed.