john_henry is a user on octodon.social. You can follow them or interact with them if you have an account anywhere in the fediverse.
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So... how do I explore Mastodon effectively? I guess I can try search and see what people are talking about. I can follow profiles. Does federation mean I can search across instances? Can I add a tab of activity on another instance? Can I explore instances without registering new accounts?
Found the instance preview tool: http://www.unmung.com/mastoview?url=cybre.space&view=local
Do people register on multiple instances?