I liked this part:
Websites typically don’t just offer integration with one service provider. It is common for a website to want to enable sharing to multiple sites like Facebook and Twitter, or allow users to upload their photos from Flickr, Facebook, and SkyDrive.
One of the challenges with supporting services that identity multiple providers on a website is what many have dubbed “the NASCAR effect,” which is when a site has so many logos from so many different providers that it looks a little like a race car with too many corporate sponsors. This practice often ends up confusing users due to the paradox of choice.
To help with this, we make it easy for your app or site to check if a customer uses one of our services before even offering the option to connect.