Actually I realised that there is quite a lot of false or misleading information around this subject. Finding out one's facebook id or facebook admin id (aka fb:admins) apparently creates a lot of confusion as facebook itself does not provide a comprehensive help page with this subject. What would you need it for, you may ask. Well, some facebook social plugins, like the facebook "Like" button for example, need it in order to work properly.
So here it is, select one of these 2 easy ways to get it:
