Many websites now include Facebook buttons and widgets of various sorts. As a user, it is worth knowing that if you are logged into Facebook, many of those buttons and widgets can be used by Facebook to track your web use and link it to your real identity.
This site has a Facebook button, as well, but it is a graphic that loads from my own server. It does not allow Facebook to add to their trove of data.
That said, Google has its own massive data pile, which this site contributes to in obvious ways like content being indexed and less obvious ways like Google Analytics visitor tracking.