In iOS Safari, why aren't the Facebook social plugin cookies blocked, in spite of the "from visited" option? In my iPad, the cookie setting for Safari is set to "from visited", which should block all third party cookies. But if I am not logged out of Facebook, then from any third party webpage that uses the Facebook Social plugin, I can see which of my friends have liked that page. That means Facebook knows that I am visiting that page, so its third party cookie isn't blocked. What am I missing here? (I use Chrome with my Desktop computer, and this does not happen, i.e. with its "block third party cookies" setting, the Facebook social plugin cannot see who I am, even when I'm logged in to Facebook.)
Facebook's 'social plugin' is iframed, and as far as the browser is concerned content in an iframe is on the domain that served the iframe, not the domain of the page that had the iframe in it.

So, to the 'social plugin', facebook's cookies are first-party cookies, not third-party ones.