The example he offers is the Facebook news feed. As it stands if you publish something to your news feed and then decide that you want to limit who can see it by changing the privacy settings, that information is removed from your friend’s updates (assuming they no longer have the privileges necessary to access it).
However, were the Facebook news feed offered as an RSS feed available outside Facebook (as we’ve often argued it should be), it would, because of the nature of RSS, no longer be retractable. Even if you changed the privacy settings and removed it from the feed, many RSS readers would already have cached or otherwise stored the post you’d like to retract.
