what's the best way to discover mastodon and/or matrix servers?

**tl;dr - what are the best ways to discover mastodon and/or matric servers?**

I have heard that both mastodon and matrix are more privacy respecting forms of social media. I am fairly new to both and would like to see what sorts of (mostly) public communities are out there. While I am not opposed to private ones necessarily, I'm not really willing to jump through many hoops to meet requirements and such... but I want to have a wide spectrum …

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