I think the biggest competitor is FB groups. I agree with @kirillpertsev that most hobbies have massive FB groups. And FB is easy to access since the members are already on there. So how do you differentiate enough? Using golf as an example you'd have to have all of the basics: forums to chat about golf, classifieds to sell equipment, etc. I think the differentiating factor might be the ability to get lessons (either from a pro, or more advanced member) as well as group meetups, bulk discounted buys for gear and golf, and possibly some online learning/course to cover the basics.

All my hobbies have FB groups with thousands if not tens of thousands of members. How do we steal these users from FB?

I met my co-founder through a Facebook group 12 years ago and ran a venture studio for 6 years. People meet their partners through dating apps and have healthier relationships. Using coaching platforms, I match mentees based on their professional interests. Why not use a matching app for your hobbies? I completely understand the need. Since hobbies are such a broad term, I think keeping it niche might be helpful. Matching apps don't work for a lot of married people. A good way to get started is to pick a niche that targets them first.

The data is a bit old , but this project shows connections/audience overlap between subreddits. This mapped all of reddit at the time so their connection criteria is stringent at 10 posts + 10 comments in both subs to qualify.

http://randalolson.com/redditviz/

A motivated individual could update the data, narrow the focus, and lower the requirements for connection to inform GTM strategy. The Fishermen you can advertise to on reddit, are they mostly hunters or TCG players?

Some quick thoughts:

As people engage with a hobby, they progress along a spectrum usually defined by the acquisition of hobby-related knowledge and skills. Different people stop at different points along that spectrum and this behavior creates the social strata within hobbyist communities.

The this/that format + hobby focus lends itself to matching people who are at a low to mid hobby level for social benefit, low/mid to high for monetary benefit, high to high for social benefit, and all to all for monetary benefit via p2p secondary market. This is a powerful combination of mechanics that builds on existing patterns of behavior in enthusiast communities. Executed correctly, this can appeal to people at all hobbyist levels.

Market detail; most apps that offer hobbyist matchmaking do so as an expansion of utility within an existing hobbyist app. The Grint, All Trails, onX, Powderhook etc.

This evolution in utility apps to community apps offers a lesson in GTM; give the chickens immediate utility.

Chicken/egg is much easier to solve if you have an intentional roadmap of communities to acquire. Each audience needs to connect withthe prior audience to accelerate growth.

Very fun idea.

It is an interesting area. Specialized marketplaces for a specific hobby could have higher growth rates, such as platomic.io. It is a marketplace and community of padel players. Many people have tried to copy their success with similar solutions but failed.

don't forget to offer a verified identity badge to help build trust and smooth connections between members. people want to extend a hand of good faith and accountability. Plus, communities thrive and grow when people control how others view and share their identity.