Mastodon has no single address to post to — every account lives on its
own server, so a share has to be aimed at yours by name.
The first time you use the Mastodon button it asks for the server part
of your address: for @you@mastodon.social that is
mastodon.social. It is remembered in this browser only.
Shift+click the Mastodon button to change it, or answer
clear to forget it.
Mastodon does not appear in the browser's own share sheet unless you
have installed a Mastodon app that registers as a share target.
