UPDATE: As of 3/11/2010, AddThis no longer uses Flash cookies.
John T. Haller, October 8, 2008
AddThis is the most popular website button allowing visitors to easily add your site to FaceBook, delicious, MySpace, your favorites, Live.com, Yahoo, Twitter and other social and bookmarking sites. On September 30, 2008, ClearSpring acquired AddThis. On October 8, 2008, ClearSpring began sending a ClearSpring Flash Tracking Object ( from http://bin.clearspring.com/at/v/1/svc6.swf ) to sites using the AddThis widget.
As with all flash tracking objects, the ClearSpring object uses Local Shared Objects aka "Flash Cookies" to track users as they visit multiple websites using AddThis or ClearSpring widgets. These cookies are not visible within user's normal privacy options windows in their browsers and can not be cleared by using the browser's Clear Private Data (Firefox) and similar privacy options. These cookies also work across all browsers on the machine as Flash stores these LSOs in a single location.
This change appears to have happened without any notification to the website owners who utilize the AddThis widget. The AddThis widget was previously quite unobtrusive and didn't utilize Flash. To date, no notification has been sent to the users of the widget.
It is recommended that all AddThis users reconsider their use of the AddThis widget in light of its new privacy implications to your website visitors as well as AddThis/ClearSprings failure to notify you of this major change to their widget. I have removed AddThis from all of my websites.
There are some alternatives to AddThis around. Here are a few: