Well they already have your IP address along with any other activity which you logged at that time - this would make a pretty strong circumstantial case, and then you would need a "reasonable" jury.
Of course using the Internet to spread the message of incitement to riot can also be used if publishing a general web page, the law doesn't take into account whether it was Facebook or not (Facebook just makes it easy for the Police Authorities to know what door to kick down). Now if it was possible to publish a website inciting other to riotous behavior, which was linked to Broadband accounts held by lets say at Number 10 Downing steeet, how do you prove or disprove that it was actually Prime Minister David Cameron who sat down one evening and published the web page?
BTW this is not at all theoretical but can be quite easily implemented, by those who have access to Number 10 Downing Street Internet access accounts. There are those who are involved in the current Phone and Computer hacking scandal who know how to implement this.