On Mastodon you can follow tags, just like following accounts.
Don't forget to tag your posts w/ hashtags. For privacy & safety reasons, text search is limited; tags are more searchable.
Most Mastodon sites have an About page w/ a Profile Directory of members. You can usually follow any member from any Mastodon site.
You can also follow people on certain non-Mastodon sites.
The more connected your mastodon site is the more posts that will appear on the "federated" timeline.
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