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.

I guess as Facebook deliberately designs the Like button to track this makes sense. Although maybe it will be ok as it will only track Facebook users, who will presumably have consented.

Facebook is legally an Irish company in Europe so cannot ignore this.