Typically, in Oracle speak, Webcenter typically refers to Webcenter Content or Webcenter portal, which are products that existed (under a different name?) prior to the Fatwire acquisition. The fatwire product was specially rebranded to 'Webcenter Sites'. From what I understand, from talking to Oracle VARs and consultants, the Sites suite isn't as common as the other two.

Browsing through the PDF, they make references to Webcenter Portal and Webcenter Content, but not Webcenter Sites.

Source: I implemented and supported the largest (in terms of assets and users) (allegedly, according to Oracle) installation of Fatwire as of a year ago. I work on a Portal/Content project now.