Referers are the new black (gold)

The referer is such a tiny bit of data. It doesn't tell more except where some anonymous web surfer came from, when he visited your page, viewed that image, watched that video.
But that's an awful lot of information.
Mining referer data is one of the most important informational bits for behavioural web analysis. The most important thing, though, is getting these huge amounts.
There are naturals, of course, like big traffic websites, that even offer linking directly to hosted content like youtube or flickr. For every video watched, the publisher of that video or picture (the one who uploaded it) doesn't even get that referer anymore. He won't know, who links to it and which links generates how many views or when they come.
In social media marketing strategies this information is extremely important to identify fans, followers or unknown affiliates.
Then there is google, with its free service google analytics. In exchange for offering some tools for analysing your web traffic, google gets all information concerning your visitors behaviour. Combined with an ubiquitous google cookie, these data chunks are not even anonymous anymore - they got a face.
Mining referer information is the link equivalent of FoF information.