Is it possible that Facebook know more about how many visitors a given website has than Alexa or Hitwise? I’m not talking about people sharing a link on Facebook and then tracking outbound clicks. I’m talking about capturing all visitors. Both Alexa and Hitwise used to both rely on browser-addons to capture this data. Since only a small percentage of users will install this add-on, they will have really rough data (and skewed towards non-technical users).
So how can Facebook then acquire more accurate data than these traditional companies? It’s pretty simple. You know that Like-button that is showing up all over the web these days? Turns out that Facebook is hosting all those images (and you cannot host this yourself as that is a Term of Service breach).
Since Facebook is hosting all those images, they know exactly (or at least could know) how many visitors you have on your website for each page you got a Like-button. No estimates, but exact up-to-date metrics. If you’re logged into Facebook and visit a site with a Like-icon, they can also track that you have visited.
The bottom line is that Facebook knows a lot more than you might think about the web as a whole.