I've seen this circulating on social networks:
I was wondering to what extent is it true. Do skin mites really get on the skin surface to reproduce? Why is it so?
Yes, it is true that these mites exist on healthy human skin, on >98% of people, no matter the country or hygiene.
However, I'm sorry to say that this photograph is not the follicle mite Demodex folliculorum. This image has been widely circulated around the internet as Demodex, but it is actually the head of a silkworm, Bombyx mori. The above version has been modified slightly from the original, but you can see the original here: https://www.sciencephoto.com/media/720736/view/silkworm-head-sem
And here is a zoomed out version of a silkworm: https://macrocritters.wordpress.com/2012/11/26/photographing-down-the-food-web-silkworms-bombyx-mori/
You can find a real close-up of Demodex here: https://www.sciencesource.com/archive/Demodex-folliculorum-(SEM)-SS2367855.html
Do skin mites really get on the skin surface to reproduce? Why is it so?
Yes, it is most definitely possible for skin mites to sexually reproduce on the surface on human skin. Do they specifically congregate on the human face though? Well, I think that varies by case; but yes, I would imagine that it does happen.
When skin mites are present, they cause a condition called Scabies. The skin mites will dig burrows within a person's skin, and the females will lay eggs. When this occurs, the skin becomes quite agitated, which will result in redness and excessive itchiness. That being said though, I believe you would be aware if a skin mite infestation were to occur. And why do they do this? Well, it's just how they've evolved to survive!
See here for further information regarding this.
Not only do skin mites have sex on your face, but my suspicion is that -- quite often -- they're having INCEST on your face. (I.e., brothers inseminate their own sisters.) Admittedly, I'm not sure about the Demodex species that live on humans, but I know that among mites as a general group, there are various species that have evolved genetic mechanisms to make brother/sister mating less harmful than it would otherwise be.
And if you're a tiny, slow-moving creature that is unlikely to travel more than a few centimeters from the spot where you and your siblings hatched out, the odds of meeting an "unrelated" mite -- say, third cousin or farther -- are not very great. Most of your potential mates are likely to be genetically close kin; and sometimes, a sibling may be the only option. Beggars can't be choosers, any port in a storm, etc.