Back when it was looked into elsewhere last year, it was found that several measures are in use. The logo itself has lines on the right side of the lettering that serve as a unique identifier that's actually a "website overlay" shown over every page displayed, generated according to user's login credentials. Since the pages couldn't be downloaded directly but only screengrabbed, that meant it's always captured with the rest. While this was the visible tracker, it wasn't the only one - multiple pgs were found to have hidden "codes" that were in the black areas of the pages, visible only if you modified the image in PS or other software and messed around with settings like brightness and/or color levels. Different but similar kind of codes were also hidden near the "page numbering" section - so if anyone took a screengrab that included that area as well, it would include those too. Codes were unique and differed between uploaders. Basically multiple hidden backups in case the logo gets edited to remove the barcode lines.
Crazy paranoid if you ask me.