Based on an old trick, Aza Raskin developed a nifty js around it to check which social bookmarking websites your visitors use. Explaining how it works, Raskin says

By using a cute information leak introduced by CSS. The browser colors visited links differently than non-visited links. All you have to do is load up a whole bunch of URLs for the most popular social bookmarking sites in an iframe and see which of those links are purple and which are blue. It’s not perfect (which, from a privacy perspective, is at least a little comforting) but it does get you 80% of the way there. The best/worst part is that this information leak probably won’t be plugged because it’s a fundamental feature of the browser.

<script src="http://aza.googlecode.com/svn/trunk/SocialHistory/SocialHistory.js"></script>
<script type="text/javascript"><
!--
user = SocialHistory();
var visitsDigg = user.doesVisit("Digg");
var visitsSlashdot = user.doesVisit("Slashdot");
var listOfVisitedSites = user.visitedSites();
// -->
</script>

One can extend this to do other interesting things like

Guess a user’s gender or tell whats on your user’s mind

Links:

Vote! How to Detect the Social Sites Your Visitors Use

Using your browser URL history to estimate gender

SocialHistory.js: See Which Sites Your Users Visit

SocialHistory.js: More Spyjax

Automatic favorites import using browser history

CSS visited pages disclosure

Using your browser history to tell what’s on your mind

Brain Cloud

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