Sex /HOT/Porn Video through Facebook or Twitter

Hi;

In my network enviorment Facebook & twitter is allowwed use in office & because they are doing so many marketing relate things in that. So it has been allowed.

Mean time some people using this facebook/twitter to watch porn/Sex/hot video in office; how can i block this only?

I cant block compltely facebook & twitter i need to block only these kind porn/Sex/hot  - content only ? Is that any way is there in Sophos ?