Partial working of rule

Forefront TMG 2010 version 7.0.9193.500

I have a rule on the TMG, called "Block Social Networks". It's first of all rules in "Web Access" applied to "Internet Standard" users' group, blocking a set of URL (called "Social Networks").

However it blocks only part of this set (Facebook, for example), but other part is allowed (Twitter, for example), i.e. rule works only half.

And ideas why?