You can't make
a product "go viral".
There are only two options:
1) Make a product that naturally takes advantage of network effects as it's core value proposition. You can only do this if the problem you are solving for your customer involves networks of some kind (sharing, collaboration, contact management, scheduling, etc.)
2) Make a product so good that customers want to refer all their friends and coworkers. And then just make sure you have functionality to make it easy. You will know it's that good by your retention metrics. Your churn will be extremely low, and you'll have natural inbound referrals already happening organically. If you're not there yet, don't worry too much about artificial growth.
Sorry, but there isn't any more magic to it than that.