Freemium is the way to go. All the essential features are free; you can pay for extra stuff like special emojis, coins(like Reddit silver/gold), or customizable profiles. It could be either a subscription or à la carte.

Simply giving something in return would incentivize people to donate more.

Unlike Reddit, the profit should give back to the communities by adding more features, paying developers to maintain open source projects, giveaways etc.