Google is making YouTube more social with a 'find your Facebook friends' feature. Yahoo contacts were also included in making it easier to find your friends from across different platforms.
"Find your Facebook friends and Yahoo contacts on YouTube: In addition to finding your Gmail contacts on YouTube, now you can find and connect with the people you know on Facebook and Yahoo (that is, those who also have YouTube accounts and have connected their YouTube and Facebook accounts through AutoShare). Sign in and visit the homepage to be prompted with an invite (see screenshot below) to sync up your YouTube account to these platforms, and from there it should be easy to subscribe to the YouTube channels of people you know through these networks. Please note that this functionality is only available if your friends and contacts have allowed themselves to be found by their email address."