It's a good point but the wording in the guidelines suggests that both the 3rd party and the site operator share responsibility to get consent (ignoring the fact that Facebook may just sidestep this, being a US based company):

"The person setting the cookie is therefore primarily responsible for compliance with the requirements of the law."

and

"Where third party cookies are set through a website both parties will have a responsibility for ensuring users are clearly informed about cookies and for obtaining consent."

It opens up an interesting liability issue. I suspect terms of service agreements for companies that provide services based on 3rd party cookies may be updated shortly.