Why does JPEG weigh more than PNG?

Question

JPEG

Hey! They say that JPEG is smaller than PNG. When saving in the program can be. But! Almost all social networks convert all images to JPEG.
I tried to upload PNG to a contact, saved it to my computer, looked. JPEG from contact - 30 kb, PNG source - 18 kb.
What's the point of converting everything to JPEG?

Answer 1

it so happened, you won’t believe there was a time when there wasn’t a single png at all, and even worse, png support in browsers was not complete and could not be shown as expected.

Answer 2

PNG is designed for images where there are many solid fills of the same color. These are screenshots, interface elements, images obtained from a vector, and so on.
Jpeg is best for full color images.
So the degree of compression turns out to be different, depending on what it is applied to. There is no point in converting everything to JPEG. By doing this, you can degrade the image quality - compression artifacts will appear.