If race was a fake social construct, then I should be able have sex with an Asian from some isolated part of China and she would give birth to a black baby.
Genetics are indeed a thing. Genetics determine things like skin color, eye color and other physical attributes.
When a non-tumblrite says "race is just a social construct", they usually mean that the definition of "race" is shaky to the point that there's no solid biological standard for where one race starts and one race begins.
It's kind of like when alt medicine people complain about "toxins" in food. There's no solid definition of "toxin". It's just variously used to mean "something scary and bad for you". Bad in what way? Poisonous? Makes you fat? What? There's no solid meaning to it. They'll use it to refer to both pesticides and hormones in agricultural products, even those two things have vastly different effects, costs and benefits.
Of course, like
@Puppet Pal Clem notes, race is still a useful way of categorizing people. It's just not biologically based. It's based in sociology and how people interpret people's physical features. It's strongly influenced by their culture.
This is anecdotal of course, but I spent some time recently with a lot of central asians; it was very illuminating. As an American, most of my understanding of what an "asian" is was limited to east asians, like the Chinese, Japanese and Koreans. I always assumed that some people lived above India, but I never really thought too much about them. So during my time recently, I learned that central asia is, racially, a very smooth gradient from the far east, to the middle east. The mongols smeared their genetics across the area.
Heh, one of the guys I was hanging out with could tell me precisely which country each person was from based on subtle little differences I never would've picked up on.
There's kind of a similar thing when comparing west african features (broad noses for example) to east african features.
