If you look at actual land, Asia.
If you look at the population, definitely Europe.
I don't remember what I was thought in geography class back in middle school in Russia, and as far as I remember people generally stuck with "Europe" because much of Russian territory in Asia is pretty unlivable and very few people compared the the European part of Russia actually live there. Seriously, just look at the map or something.
And then there's all the apparent prestige of being European that some people find a big deal, somehow.
Although perhaps the right answer is Eurasia, I've never been a big geography nut back then. But still, as concentration of population goes, it's Europe.