Once in a while customers actually tell me that they've referred someone, so it must be happening. It's nice to have customers say they would tell their friends in a survey response, whether or not they actually do. :)
Also, the best part, by the way, is you can then contact those who said "needs improvement" and ask them what happened. The article talks along the lines of you "can" do this. I say, you HAVE to do it.
do you know how many many of your users found out about the service via a recommendation/referral?