I recently joined - http://area51.stackexchange.com/proposals/3659/the-great-outdoors
I sent the link out to all of my friends. A few joined. Referral count 0. I decided to get my wife to do it. Referral count 0. She un-committed and recommitted using the link (again), referral count 1.
My conclusion right now is that something in the newbie account creation loses track of where the referral is coming from. I watched her through the whole process and she used the link to get there to sign up, but the referral only counted when she re-committed after the fact.