Pro tip for anybody on a custom Mastodon server who can't figure out how to follow somebody on mastodon.social:
Copy the URL of that person's profile and paste it into your own Mastodon instance's search bar. That'll give you a way to follow them within your instance.
The reason for this is: mastodon.social is using a newer/feature version of the Mastodon code.
Thanks to the excellent support at masto.host for telling me about this. Never would have figured it out on my own.