Sample code showing how to allow the Facebook Crawler to bypass the Force Login plugin

wp-force-login-bypass-facebook.php

<?php
/**
 * Bypass Force Login to allow for exceptions.
 * 
 * @param bool $bypass True to disable Force Login. Default False.
 * @return boolean
 */
function my_forcelogin_bypass( $bypass ) {
  // Allow the Facebook Crawler to access the site
  if ( strpos($_SERVER["HTTP_USER_AGENT"], "facebookexternalhit/") !== false || strpos($_SERVER["HTTP_USER_AGENT"], "Facebot") !== false ) {
  	$bypass = true;
  }
  return $bypass;
}
add_filter( 'v_forcelogin_bypass', 'my_forcelogin_bypass' );

In case you need to allow other social networks to bypass Force Login, here are some other crawler user agents to reference:
https://gist.github.com/Paratron/8676518