You can always contribute by crawling ipv4 range searching for http servers that don't necessarily have a domain pointing at them and posting a list for us when you're done crawling it merely once :)

I'm assuming you're referring to Facebook groups or similar walled-garden web presences? If so, I'm assuming indexing those is an explicit non-goal for OP, and I'd agree with that. The walled gardens are very hard to index (often actively hostile), which means the returns would be rather low. Plus, most such places have searches of their own if you're really interested in finding something there.

In my experience, part of the problem is that often (usually?) the search on those places is nearly useless. For example, I shudder to think what will happen if old.reddit ever stops working.

True, but for reddit adding "reddit" to a google search does just fine. Not so much for the single-purpose websites marginalia focuses on.

Well the literal point of the tool is to show that there are websites outside of the walled gardens. There are plenty of link aggregators already. Hacker News is one of them. Reddit is another. Twitter is a third. Google is a fourth.