[xwiki-users] Problem with action_xyz parameter in URL

Hi I have a problem with a URL containing a parameter of the form action_...=... The page does not load with this parameter. For an example see the xwiki.org start page ( http://www.xwiki.org/xwiki/bin/view/Main/WebHome?action_xyz). The parameter action_... seems to be used for something, but I couldn't find anything on what exactly and on how I can work around that. Unfortunately this URL is created by an external source (Facebook), so I can't change it. Can somebody point me in the right direction? Thanks, Edo