As such, trackers in 3rd-party advertising network code probably won't count as necessary; it's necessary for the ad network, but it is not necessary for the website displaying the advertisements, as proven by ad-blockers.
Likewise, the facebook like button and other "plugin" components aren't "necessary", they merely add value to the existing content / features, and must therefor be entirely opt-in.
At least, that's my understanding, which is entirely too facile to be taken as legal advice.
EDIT: As petercooper pointed out below, in addition to proving necessity, you must still also prove in court that you have balanced the necessity against users' rights and interests. As such, I no longer feel confident to say I even have a clue as to what is legal and what is not, for FB or for anyone else, until there's a big enough court case to set precedent.