rel=me links allow cross-linking of user's identities. This behavior is
used by software to determine if user controls profiles on two different
sites. For example Mastodon uses it to display verified badge on links.

Both Twitter and GitHub automatically add rel=me to links added to
users' profiles.

See: https://www.zylstra.org/blog/2018/10/mastodon-rel-me/
See: http://microformats.org/wiki/rel-me