Depending on the Terms, analytics of your data might be valuable. What data do you have, what could be learned from it, and who would pay for that knowledge?
Depending on user stickiness, you could move to a paid model. And depending on your definition of "monetize" this wouldn't need to be prohibitive, e.g. $1 per year, or $1 for lifetime/device membership. Of course, we're all spoiled by "free" services at the moment, so obvious caveats apply.
If you own the platform and the code, could you make a corporate or white-labelled version? Would Megacorp pay for a branded version for their 1000s of users?
Could you exit and sell to Megacorp? Are you screwing over your users if you do this? Is there some sort of halfway-house alternative, e.g. a big player in a relevant field who would pay you to continue development and support? See recent HN conversations about monetising open-source projects for discussions about this sort of model (whether yours is open-source or not these conversations might still be relevant).
Could your users monetise their own content, and pay you a slice, kinda like substack et al?
You can make it completely optional and for the most part not upset users who don't buy them.
There is basically no upper threshold on how much you can charge. You want to have really cheap stuff too obviously. Something for everybody.
In gaming this trend of being able to buy cosmetic items has also been developing. In some games users pay hundreds of dollars for one cosmetic item.
Rather than counting on your own creativity to create the digital goods you can also create a marketplace for user created goods and then take a fee off the top.
You could do all this with crypto, or NFTs. There would be nothing wrong with doing it that way. But you could also just store that stuff in databases and it would work about as well for these purposes.
I think other market places might let you do that too.
The challenge is you already set the expectation that everything is free. Advertising is probably the least disruptive way to monetize. I would reconsider advertising. There’s a reason it’s a popular business model.
So the only way to make one work without ads is to have a philanthropist fund it. Another option might be for it to be set up by some research organisation which wishes to mine certain aspects of social interactions.