You probably found a bunch of people with niche (if any) interests and/or friends from Twitter or other social media who are active enough for your feed to be populated. A lot of people use twitter to follow news, celebrities, journalists and shitposting accounts with large following as there's lots of discussions under tweets with massive reach. Most people don't interact themselves at all but just scroll. Mastodon doesn't have these big accounts as of now so there's not much activity for the majority of regular twitter users.
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