i think that the corrections you perform in lightroom will not be applied to the original files. So what i always do is to export what i want to upload to another folder if i am just using lightroom for adjustments...
So export the files with presets in lightroom might help? it does for me.
it is because when you view it in your photostream it is in MEDIUM size
if you view your full size photos with such restrictions they look "compressed", but they shouldn't be looking softer, they should be looking oversharpened and too "hard"
but if you look at "all photos" tab, the full size will be shown and the proper look is out.
it is the same photo, just resized automatically by flickr. i think around the eyes the wrinkles suddenly explode in unison for the second
the second is what you see, width around 500 pixels.. so if you want best sharpness in your photostream save with 500 pixels lor.