Facebook (and the exporter) reduces the size of your file but mostly because it reduces the size of the picture. That doesn't explain why colors fade: red is red, it shouldn't become light red or pale orange and it has no effect on the size of your file.
Facebook don't just reduce the size of the file. They also chuck away virtually all the metadata, and that explains exactly why colours fade.
Have a look at this:
I uploaded an image to FB, then downloaded it again.
On the left is the Get Info pane for the image that's been “processed” by FB, on the right is the same image from iPhoto.
The downloaded image is missing a colour profile.
Unfortunately, on computers, cameras, printers etc “red” is not “red” because every part of the chain can have a different definition of “red”.
The point of colour profiles is that they are a way of agreeing between the various devices and software the definitions of the colours. So, when we use a particular profile all the elements of the chain of devices and software know that we're using “that definition of red”.
Browsers like Safari, for instance, respect Colour Profiles no matter which ones are used, other browsers assume that all pics have an sRGB profile, or simply ignore Colour profiles at all.
If a file has no colour profile then what happens to the colours is pure chance.
So, if you strip out the definition of red from the file, it can come as no surprise that what you see may become light red or pale orange.
I can see why an organization like FaceBook would want to compress the files to save space on their servers. But the colour profile is an infinitesimal part of the metadata of the file. The fact that they choose to chuck that away with all the other stuff is a statement about how seriously they treat your photos. Other sites of comparable size - Flickr, for instance - don't.
Regards
TD
