if we could do something like a “readonly” mode for specific (facebook) instances.

Read only mode might not be too bad, as long as Facebook is diligent in minimizing their environmental impact. If they cached heavily, like API calls and content delivery, that could avoid crushing local instances with drive-by or viral traffic.

However, I think that would be counter to Facebook's own monetary incentives in bolstering their own user engagement and account retention. This may also impact our instance's own sustainability outlook, as users hosted on this instance have the greatest incentive to donate to keep the lights on. If the majority of users are remote, then that incentive to donate is one step further removed.