Looks like a new page has been added to the official Radiohead website. If you go to the “scrapbook” area and click a few links, you’ll notice a “leave” link at the bottom right. That page (click here for it) describes what’s going on with those buttons everywhere.
Most of the sites that most people visit throw you cookies which you accept without realising, unless you have configured your web browser to alert you about this. This information – the pages you visit, the choices you make, the interests you express, where you come from, the frequency of your visits, and so on – is extremely valuable. There are hundreds of companies, employing countless people, that exist for this very purpose. We don’t sell the information that we get from you – instead, stupidly, we use it to make graphs. For no reason at all, except perhaps to alert vigilant visitors to what’s being done with the information they volunteer every time they open up their web connection.
(thanks to Phil)