I'm no expert on sales/marketing/distribution either, but... given that you run a paid service, each customer has a tangible lifetime value. So you can offer a

I'm no expert on sales/marketing/distribution either, but... given that you run a paid service, each customer has a tangible lifetime value. So you can offer a referral commission to existing users every time they get a new customer to sign up. Groupon, Dropbox, and tons of others do this.