Take a look at this article about DropBox.

http://www.referralsaasquatch.com/dropbox-customer-referral-program-by-the-numbers/

The Company was able to have a 3900% growth rate over 15 months. This outstanding growth was created by implementing a successful referral campaign. If you can create a referral campaign that gives your users something in exchange for their efforts they will feel more compelled to invite people onto your platform or service. This type of growth campaign can be difficult to implement, but if you are at a stage prepared for growth, then you should be able to seamlessly integrate new subscribers onto your platform without too many (if any) hiccups.

Before you begin a referral campaign, remember that customer acquisition can be expensive. If you're only motivation behind the growth in customers is financial return, then consider whether your pricing structure is optimal before you rush into a referral campaign. You may be able to substantially increase your revenue by increasing your prices or looking for creative ways to reduce your overhead. Either way, when you are ready for rapid growth on a subscription-based service, (freemium or not), referral campaigns can be one of the most cost-effective ways to grow your customer base.