[johnnygood]

It doesn't break them. Safari doesn't accept cookies from across domains (by default). Safari still sends cookies to iframes across domains.

Even if Safari blocked sending cookies to other domains, it would still be possible for both commenting and the like buttons to work. On a low-tech level, rather than working by Ajax (which is how I assume they work - I haven't actually used them), they would work as a standard link or form to Facebook's site. Once not in the iframe and on Facebook's site, it would realize that you were logged in and rather than asking you for your email and password would record the like or comment. In fact, when you try to press a like button not logged in, a small window pops up asking for your email/pass so that it can log you in and do just that. It's easy to imagine that the window would pop up, offer a little "thank you for your comment" confirmation with a close button.

Facebook could also decide that it wanted to use Flash cookies which would work if Safari blocked sending cookies to other domains. Safari isn't in control of whether Flash is loaded in an iframe and a Flash cookie is requested from another domain. This is part of the reason why it's more significant on the iOS platform. Advertisers do use Flash a lot and that usage can be used to set a Flash cookie that the browser doesn't control and Flash doesn't have a setting to disallow Flash cookies from sites you don't visit.