I'm shooting raw and I'm on ubuntu. This happens mostly with pictures that have a lot of black or that are very dark. On my screen the picture looks fine (as intended) but when I upload it to 500px or flickr, a lot of detail is lost from the darker areas of the picture. However, if I save the image back to my computer and open it, it looks fine. An example would be this one where I can't see any detail in the sand when I open it in the browser, but after downloading it it looks fine.
[This happens both on Firefox and Chromium]
[It seems it also happens when I open the image with Firefox, even if it is on my computer]