YouTube has announced an upcoming change that affects "friends" associated with YouTube account. When the change takes place, any existing "friend" connections will be automatically converted into Address Book contacts, and when you visit YouTube.com you will be given the option of creating subscriptions to any or all of those accounts.
"Around December 14 in production, we will automatically translate requests to the existing friends activity feed's URL into requests to the new subscriptions activity feed, rather than just returning an empty friends activity feed. While this translation will take place transparently behind the scenes, we encourage all developers who are using the friends activity feed to explicitly update their code to point to the new subscription activity feed as a best practice," informs YouTube API Team.
"One specific feed that's exposed via the YouTube Data API is no longer relevant: the friends activity feed. The users of your application may have converted their old friends to corresponding subscriptions, and in that case you can make use of a new feed in our staging environment that contains activity updates from the accounts you're subscribed to," the team explains: https://gdata.youtube.com/feeds/api/users/default/subtivity
"There are also API calls related to YouTube contact management that would accept either an email address (in a
<gd:email> element) or a YouTube username (in a
<yt:username> element). As a side effect of this migration, you can only use
<yt:username> for these calls, which is what the API's documentation has recommended doing all along. An attempt to use
<gd:email> will lead to a HTTP 400 error response," the YouTube API team added.