Why does Facebook correlate strongly with Pornhub on Google?

Who claims to be the "facebook of porn"? reminds me of an xkcd comic ...<p>Edit: <a href="http://xkcd.com/624/" rel="nofollow">http://xkcd.com/624/</a>

Not just once, but in other variations too (e.g. "prn hub" and "orn hub")