There's a lot to be said for thanking your referrers ... and doing it publicly.
The fact that you should thank a referrer goes without saying. They've just done you a favour and you should make them feel warm and fuzzy about it and make them want to do it again. Doing something unexpected at this point is a good idea. That could be anything from giving them some free product or an extra discount to sending them a personalised email from the CEO or a senior team member. You don't have to spend money to make someone feel loved.
The reason for wanting to thank them publicly is two-fold. First, it'll make your referral sources feel appreciated. Secondly, it'll reinforce your company's referral worthiness. There's no better way to encourage others to do something than by showing that they're not the first and that they're joining a popular group by referring.
There are obviously different ways of doing this.
In the Referral Engine, Johnn Jantsch refers to an example in the US of an opticians 1.
They offered 100% refund if a customer referred four new customers within a year. They gave out referral cards and then sent a cheque for 25% of the order value every time a referral came in. Then, when they got four 25% off vouchers they became members of the "100% club" and their photos were displayed on a wall in the office. The scheme worked very well and customers became excited about "winning the game". This is a great example of thanking publicly. And of course this could translate very easily into the online world with a page called the "100% club wall" or including a section in a regular newsletter thanking the new members of the 100% club by name.
Similarly, social media offers a great opportunity to thank customers who have referred you. Doing so not only makes them look proactive but allows you to make a statement about the fact that others are willingly referring you. Having constant "thank you"s to your referrers in your social media stream is a great way of raising awareness of refer-a-friend programmes and encouraging others to refer.
Doing this well requires thinking it through and determining what will work best for your own situation but there's no doubt that the "thank you" is important and if you're not doing it publicly you're missing an opportunity to spread your referral programme.
1 p214, The Referral Engine by John Jantsch