Furthermore, let me remind you that Freebitcoin already has more than 50 million users, and it seems to me that now it's hard to find users who are fond of crypto gambling and still haven't heard about this site.
It's harder than it was in the past, but let's not forget that there are new people being born and growing up every day. So it's not like we'll ever achieve a state of everyone being familiar with it.
My personal experience with referrals is that was fairly easy to get new people to sign up, but most of them would not stick around for a long time.
But it's been a while since I was actively promoting freebitco.in, it could be easier now, when the site has much more features and incentives.