I think what would be useful would be a transparent system where the instance owners would keep a record of how many users / interaction they're getting and how much it's currently costing.
That way if they say they're supporting 20,000 users and we need a VPS instance that costs X a month for the current number of requests, you can somewhat forecast the need for expansion
I get the idea isn't to make mega servers like Reddit, but if people like the UI / experience of a single instance it's hard to tell them to go elsewhere. Having transparency about actual costs would be useful, you could have a little widget on the sidebar or footer showing how much it currently being donated.