The social share link for Stumbleupon gets redirected to a 'Learn More' generic page for 'mix.com' instead of a login page or in processing the post request. I presume that Stumbleupon has been acquired by Mix.com.
The described behavior is not longer valid. What happens now is that you get prompt into signup to mix.com and after doing that the share works as expected.
I guess that they noticed that they broken a ton of share buttons and they amend the error.