It is difficult to determine the underlying issue when you are not able to replicate the issue. Since Facebook is reporting that it is receiving a 503 when reaching your page I would recommend taking a look at your web server’s log files. If your server returned a 503 error to the Facebook service it would be recorded in your log.

If you are running Apache:

On Ubuntu/Debian
/var/log/apache2/error.log

On CentOS/Fedora
/var/log/httpd/error.log

If you are running Nginx:
/var/log/nginx/error.log

All errors your server returns are recorded here by default usually with additional information about the error encountered.