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.

Damn. I thought these kinds of things only exist for a fully automated society, where AI and robots replace all the jobs, and everybody gets UBI. Some people might have nothing else to do, so they voluntarily go to (role play) work so they have a predictable life filled with purpose.