add_filter(
   'friends_mastodon_instances',
    function( array $string_list ) {
        // Your code here.
        return $string_list;
    }
);

apply_filters(
					'friends_mastodon_instances',
					array(
						'mastodon.social',
						'mastodon.online',
						'fosstodon.org',
						'hachyderm.io',
						'infosec.exchange',
						'indieweb.social',
					)
				)