Yep I've got four signups total, and only one was a friend. A lot depends on if you are the first tracker the visitor has clicked on, because if you are not, than you won't get the signup.
I always use my trackers when sharing my hubs on social media sites and this seems to be where I get most signups from. But it is also important to include the referral in your hubs in case people get to your hub through search engines.
It is sort of a cumulative process, like writing on hubpages, where the more you do it, the more signups you will get. I try to use my referral every time I link to anything on hubpages because you never know when someone might click a link and choose to signup.
But don't make a big deal out of getting signups, you make almost nothing from it unless they write a few hundred hubs or you get a few hundred signups! But it is fun to see people join.