I doubt at this point there are many people who don´t know adblockers exist. We´re talking about 150 M users. Even if you don´t use, you know someone who does.
There are a lot more people, especially the older and younger demographics, that are not as savvy and are not aware that ad blockers even exist. A good portion of the rest might not trust ad blockers or get frustrated if they install it and it causes some kind of issue. The numbers people bandy around range from 10% to 20% of all internet users using adblocking software.
That's why it hasn't been a problem up until recently.
YT, not only offers you billions of videos, but also the ability of subscribing to original and independent content from all types of people. FB doesn´t offer you that.
FB doesn't offer you that
yet. There, I fixed that for you.
I am one of the relative few here who wholeheartedly believe that Facebook will develop a video platform (like they are planning), invite more and more of the bigger content producers and YT channel creators to share revenue, and eventually be a strong enough force to endanger Youtube's stranglehold on the video market. There will be growing pains along the way, just as there were with Youtube as they worked out some of their issues earlier on (content ID, monetization, false flagging, invalid click detection algorithms, etc) but I'll say by 2020 Facebook will at the very least be a major player, and possibly even being the market leaders by then.
Ultimately, as far as ad blockers are concerned, it doesn't matter if people are driven to FB. FB will have to have their own way to deal with those same ad blockers, although I am quite sure that there is a much lower percent of FB users using ad blockers simply due to the age demographic.