I found myself bamboozled by the site too. The clip below (from
http://www.stumbleupon.com/tag/word-games/) shows some trace of BBB's presence, but I'm blowed if I can understand the system. I wouldn't worry about it.
As you can see in the attached chart, there was a momentary surge in visits to the Letterbox page, presumably because the link was shown when anyone clicked on some tag, like word-games, or card-games(?) But the devotees of StumbleUpon soon shifted their focus to new sites. I have seen this sort of thing before, when one of my sites is mentioned on a site that has much higher traffic. It never seems to have any lasting effect - presumably a few hundred people clicked on the link, took one link and then hit the Back button and moved on.
Sorry to those who wasted some time trying to fathom out StumbleUpon. That certainly wasn't my intention - I want everyone to waste all their time playing Chi and Letterbox.