My favorite phrase as it relates to this is: Fake it, til you make it!

The question I would ask is, can you be hyper-focused on a specific small group of users, and seed content specifically for them? Facebook started with one specific school, and seeded itself with information on classmates. When people joined, they found it engaging, and kept coming back.

Identify a group in your city and approach them about using your platform to solve a problem they are having. Get them sold on your platform. Get them onboarded. Work with the leader of that group to ensure there is content and activities for those users.

Use that group as your alpha group, and learn from them what works and what doesn't. Validate your platform is a solution they are happy to use.

Then repeat the process with a new group.

It will be a very manual process to start, but that's how you'll get those initial users on your platform. Just keep grinding away, and it will start to grow.