Using data collected by its Ghostery browser plug in, Evidon monitored tracking cookies for more than 1.6 million users. The most common tracker, found on more than 70 percent of all sites, is Google Analytics. The second most common tracker is Google Adsense. The rest of the top five is filled out by Facebook’s social plug-ins, Google+ widgets, and Facebook Connect.

So basically Facebook and Google are the biggest trackers on the web.