users "land" on this page and you want them to have no other option as taking your action (you don't want them to "land" somewhere else). So you keep any distracting menu, footer links, mostly even share links away from your action page. Your supporters should first take part in your action and then share, not having the option between sharing and taking part ;-)
If it's not a action site or any other form of landing page and content is missing, it might be another reason.
Check out other forumposts,
or contact firstname.lastname@example.org