Test them all. put $100 in each channel and see what sticks. For example, I'd find a small dedicated community on reddit like https://www.reddit.com/r/DunderMifflin/ or https://www.reddit.com/r/survivor/ or https://www.reddit.com/r/nba/ and get those communities to use it. Make great memes and link to your app. Then I'd create accounts on your platform to seed it, like reddit to show that the community is alive. Just my 2 cents...