Actually, flickr has an auto-sharpening preset so I upload in flickr full res as if it's social media-sized, it comes out way oversharpened. ONce I post from flickr, it's always looked good to me.
I used to have the same problem. I was using Google and linking in FM. Then I subscribed to FM and have been uploading files directly. They are much sharper and there's no density or color change.
Your Black is clipped in the Shadows for both pics which visual effect is lack of contrast/sharpness.
- The pic with the boy has noise. As this was shot at ISO640, I assume you have either heavily cropped or added noise via a color tweak or plugin effect.
- The pic with the girl is sharper but seems to have received some color tweaking as again your Black is clipped and the white is blown out on her bonnet right.