The main Facebook page for your site is assigned according to the site's main broadcaster. You can assign the main broadcaster by going to Websites > Site settings. If you need to change the Facebook page linked to your broadcaster, go to Communication > {broadcaster} > Settings > Facebook and connect the page you want for your nation. You will have to be logged into Facebook as someone who has permissions for that page.

If you don't have a Facebook page connected to the main broadcaster, people will still be able to like the page on Facebook and share the link to the nation there.

Sometimes if you change your page settings, Facebook won't immediately catch up and it will continue to highlight an old headline from your site. If that happens, you can update your site's Facebook settings by running your URL through Facebook's debugger.