Excerpt from the article:<p>A friend-of-a-friend tipped us off over the weekend to a rather clever way that Facebook is taking it one step further: non-existent sound files.<p>You can see it yourself by opening just about any email sent by Facebook in the past year or so (and possibly even earlier) and looking at its raw HTML. Somewhere in there will be a bit of code that looks like this:<p><img src=”<a href="http://www.facebook.com/email_open_log_pic.php?mid=XXXXXXXXXXXXXXXXXXXXXXXXXXXX”" rel="nofollow">http://www.facebook.com/email_open_log_pic.php?mid=XXXXXXXXX...</a> style=”border:0;width:1px;height:1px;” /><p><bgsound src=”<a href="http://www.facebook.com/email_open_log_pic.php?mid=XXXXXXXXXXXXXXXXXXXXXXXXXXXX”" rel="nofollow">http://www.facebook.com/email_open_log_pic.php?mid=XXXXXXXXX...</a> volume=”-10000″/><p>The first bit, the img source line, is Facebook’s tracking pixel. It tells the mail client to ping Facebook’s server for an image that doesn’t seem to actually exist. Facebook’s server sees the request, and can use the email’s unique 28-character identifier, shown above as a series of X’s, to flag that email as having been opened.