Looks like this rule works to blocking facebook domain, when not visiting the facebook domain (ex.: "like/share" etc. buttons on other pages then facebook):
! don't allow facebook outside facebook..
||facebook.com$domain=~www.facebook.com
||facebook.net$domain=~www.facebook.com
||fbcdn.net$domain=~www.facebook.com
facebook.com###appsNav
facebook.com###pagesNav
facebook.com###groupsNav
facebook.com###pagelet_friends_online
facebook.com###pagelet_bookmark_nav
facebook.com###pagelet_rhc_footer
But how can I block completely twitter / g+ / etc sites with adblock rules? (Is this really effective, or it only makes me not to see the fcb buttons/etc.)?