site1.com might include embedded images that are hosted by site2.com, so may site3.com, site4.com, site5.com and site6.com Now site2.com (aka facebook like but

site1.com might include embedded images that are hosted by site2.com, so may site3.com, site4.com, site5.com and site6.com Now site2.com (aka facebook like button) knows that you have visited both site1, 2, 3, 4 and 5. You can work around this in some ways by disabling "third party cookies" but this breaks certain features, such as using your facebook identity to post comments on other sites, so sadly all browsers enable this by default.