The major problem is not how to catch a guy who is publishing his willingness to kill infidels and ready to die for the cause. This is an easy pattern recognition.
The major problem is the ideology of the western society based on a postulate that all people have the same desire - eat well, live in good house, watch movies, educate children and a postulate that any religion as good as any other religion. Under those postulates it is intrinsically impossible to make a separate line between a moment a person is a jihadist extremist and the moment he is a devoted believer. No smart pattern recognition will help any government accepting a postulate of equality to find a bad guy without touching a good guy.
It is not a technology problem - it is an ideological problem