Find a person's profile across 350 social media sites
I think that anyone developing a social media app for the 2020s will need to make a conscious decision to not let users pick their own unique identifiers. You can't expect users to protect themselves, because you don't know what dangers might exist in the future for them that do not exist today.The other thing you can do is not allow your API to iterate over the set of all users by user-set identifier. It's an extreme flaw of the telephone numbering system. You would think by the 2020s we'd have learned to use a unique UUID designation for every user for any public facing APIs. If users want to link to it on their public pages, that would be on them.