How important is it to follow branding guidelines? Are they required?
I've seen really important websites like Gallup freely modifying logos such as the facebook one, clearly against their guidelines, but it doesn't look like they are getting in trouble.
gallup isn't even in the top 10k on alexa... Either way, the most a company can do if you don't follow their branding guidelines is usually to forbid you from using their branding (which they own.)
Some companies may care more than others. Most companies understand that being ubiquitous is the over-arching goal, and having consistent branding is just one component to achieving that. If an important website that will help you establish your brand slightly violates your branding guidelines, you would typically ask yourself if it's worth forbidding that website from using your brand -- will that actually help strengthen your brand? Likely the answer would be "no".
Also, from what I can tell gallup only modifies the facebook icon very slightly (white on black background in the footer) to unify it better with the style of their website. But maybe I'm not seeing the specimen you were referring to.