Why did joining Facebook create ads for me?

I have been using Adblock Plus with excellent results (Firefox 52.0.1 and Win XP). I have never had an ad on the SD site.

I joined Facebook about a year ago to be able to post photos to one history page. Then I quit FB. Recently I have been trying to track down a historian who has posted on various blogs and forums I am interested in, they they are all dead pages and none of his old emails work.

He is on several historical foundations on Facebook. So I rejoined and sent a friend request. Haven’t heard anything yet. I can’t find a way on FB to just write to him to explain who I am. You have to be friends first? Is that right?

Anyway. Instantly, where ever I go on the Internet I am absolutely inundated with ads. The SD is the worst. Top ads, middle ads, bottom ads. Even if I stop the page from loading some pages reload by themselves.

WTF? How did re-joining FB allow all these ads to sneak by Adblock? How can they tell? Why is Adblock defeated? I can create a filter for some of them but each time I go to a new page a new bunch crops up.

Dennis