Now when I see the "stumbleupon friends" threads here, I can't help myself but wander why the hell bother?

The easiest way to get stumbleupon friends, is to become a fan of a few hundred people.
That's really not hard, just pick a random page with lots of stumbles, open the profiles of all the people who stumbled it (this also makes sure those people are active stumblers) in separate tabs, and then one by one click the "become a fan of this user" button.
Heck, just write a script that does all that.
From my experience, more then 10% of people who you are fan of will eventually add you as a friend, most of those will do that on the first day.