Don't use Site Icon as fallback for og:image

/**
 * Modify Jetpack's presentation of Facebook's Open Graph tags
 */
function eth_jetpack_og_tags( $tags ) {
	// Site icon is a bad fallback; see https://eth.pw/a0
	if ( is_string( $tags['og:image'] ) && has_site_icon() ) {
		$site_icon = get_option( 'site_icon' );

		if ( $site_icon ) {
			add_filter( 'jetpack_photon_skip_for_url', '__return_true' );

			$site_icon = wp_get_attachment_image_url( $site_icon, 'full' );
			$site_icon = pathinfo( $site_icon );
			$site_icon_regex = '#' . $site_icon['filename'] . '(\-([0-9]+)x([0-9]+))?\.' . $site_icon['extension'] . '#';

			if ( preg_match( $site_icon_regex, $tags['og:image'] ) ) {
				$tags['og:image'] = 'https://s0.wp.com/i/blank.jpg';
				unset( $tags['og:image:width'] );
				unset( $tags['og:image:height'] );
				unset( $tags['og:image:secure_url'] );
			}

			remove_filter( 'jetpack_photon_skip_for_url', '__return_true' );
		}
	}

	return $tags;
}
add_filter( 'jetpack_open_graph_tags', 'eth_jetpack_og_tags', 99 );