Adding Information to Maintenance mode message
If you just wanted to add some text I'd say go into the admin settings of /admin/config/development/maintenance and update the text but if you want to add dynamic code like a facebook Iframe like button you're probably better off adding the code to a custom maintenance-page.tpl.php in your theme. Grab the code from Facebook and twitter and paste.

Just add it below the line that reads

&lt;?php print $content; ?&gt;


Drupal api reference - https://api.drupal.org/api/drupal/modules!system!maintenance-page.tpl.php/7