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GitHub User: dguzzo
Damien, that is a valid question. A potential answer might be that Ryan was merely using those share icons as an arbitrary and easy-to-grasp example. But, he may have left them not-sprited on this site for accessibility reasons; images that have a function (merely than just aeshtetic) have the crucial alt attribute for screen-readers and when an image is broken or not loading for whatever reason.
If you Google search for something like "css sprites alt accessibility" you'll find plenty of arguments for what best practices are when it comes to Spriting, but the consensus seems to be "don't Sprite everything" as well as numerous different approaches to hacking emulation of alt tags.