You can block the social networking category, and add the URL to the specific page you want into a custom URL category, and allow that.
There are many different ways to achieve this. It all depends on what you need.
thank you Mr Vance for the quiq answer,
actually i did it using a regular expression and it works , but the page is not fully displayed only the last updates are there. moreover , it doesn't work for the unauthenticated facebook users, so
we have to authenticate to facebook , then save username and passord to the browser for this policy to work
any idea please?