Especially if you belong to social networks.
For example, Facebook or Twitter know when one of their members reads an article about filing for bankruptcy on MSNBC.com or goes to a blog about depression called Fighting the Darkness, even if the user doesn’t click the ‘Like’ or ‘Tweet’ buttons on those sites.
For this to work, a person only needs to have logged into Facebook or Twitter once in the past month. The sites will continue to collect browsing data, even if the person closes their browser or turns off their computers, until that person explicitly logs out of their Facebook or Twitter accounts, the study found.
From a report at ‘WSJ.com.