Let's not forget it's not just math involved here. You have to think of psychology as well. It's been shown that if you stick people in a group, it changes the way they approach problems. For instance, if everyone else in the group is giving the wrong answer, especially if they seem emphatic about it, ("OMG this is sooooo easy. It's totes 0," says annoying Facebook girl) then others will just agree with them even if they know it's wrong. I realize you can say the internet is different than a group of people irl because you're anonymous, but on Facebook, you're not really. Most people do use you their legitimate name there and what you say will be connected back to you.
TLDR: Wanting to fit in > Wanting to be right.
However, I will admit there are probably a lot of people who just don't know the right answer, but think they do. However, any good math teacher would never give the problems listed on that site as an assessment question because they are unfair/confusing. Give someone (5+9)/7x2? Fair assessment. Give someone 2+2+2+2+2x0? Not a good test question.
As for me, I want to be a teacher, so I will have to do some basic math and statistics, but nothing too complicated and most programs today offer those processes with a click of a button. I got up to Calculus I in high school, and I took a stats course and a couple economics courses in college. I always did ok in math. Usually Bs. However, I will say that I do not have mathematical intelligence. I think that involves some creativity of understanding how you can work numbers around to get the answer. I can plug and chug fine, but anything slightly different from the formula and I have no idea what to do.