"When a person is logged into Facebook and visits a partner site that is using a social plugin, such as NHS Choices, Facebook can see technical information such as a person's User ID, IP address and operating system," said Sophy Silver, head of PR for the social network.
"This is industry standard data that helps us customise the experience on the partner site for the person, such as showing what friends have liked or recommended. Our highest priority is to keep people in control of their information on Facebook."
She added: "If a person decides they do not want a personalised experience on a partner site, they can log out of Facebook and we will not receive their user ID."
UPDATE: Google responded to our request for comment, claiming it was "not the case at all" that the company was tracking user activity.
A Google spokesperson said: "Google Analytics helps site owners understand how visitors find and navigate on their websites. The service only reports on aggregate visitor data in anonymous form and does not collect personal information like addresses, names, or credit card numbers."
"The data collected by Google Analytics is not used by Google for anything other than reporting site usage back to site owners who use Google Analytics and helping them improve the efficiency and usability of their website."
