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@jmuscara @jean @help To follow a person, they need to host their content on their own domain or subdomain. Like example.com or example.tumblr.com. People hosting content under directories like tumblr.com/example/ can't be followed. Also, there needs to exist a web feed (RSS, Atom, or JSON). Neither is true for the two examples you provided, unfortunately.
If you share a person you can't follow on Mastodon, I will try to figure out why.