Sorry, I should have made that more clear. Facebook knows about it. To get the plug-in, the browser has to hit the Facebook servers so all the context has already been passed along. I will edit. Thanks.
1) "Social plugins are engineered in such a way that no personal information is shared with websites using them."
2) "When you and your friends visit an instantly personalized site, the partner can use your public Facebook information, which includes your name, profile picture, gender, and connections. To access any non-public information, the website is required to ask for you or your friend's explicit permission."
This seems contradictory unless "personal information" is taken to mean "non-public information".
Or are "social plugins" and "instant personalization" not the same thing?
I'd like to hear some backup of this theory though.
The big winner here is Facebook. Comments, likes and all that stuff all make they way back to the Facebook mothership. The external websites may or may not get access to that info.