Safari help says "Select to reject cookies from advertisers and from “third parties”—websites other than those you open. This might help prevent certain advertisers from storing cookies on your computer."
This might mean that they reject third-party cookies from being set, but not from being read. That is, once you get a cookie (e.g. signed in to Facebook), every other request to Facebook (even if it's through the "Like" button) will also send your cookie.
The flag that appeared in the latest Chrome beta specifically disables sending of cookies to third parties, even if you have them.