
An obvious privacy trade-off is that the extension sends the list of the pages you visit to Google's servers, but this information is not associated with your account. "Google doesn’t keep a persistent record of your browsing history as part of the process of showing you a +1 button or otherwise use the fact that you personally have visited a page with the +1 button. Google may keep some information about your visit, usually for about two weeks, to maintain and debug its systems," explains Google.
{ Thanks, Kristian. }