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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.