How To Build Virality Into Your Business

Clearly, the Internet has revolutionized the world. Millions jump online to look for information, to make a purchase or to simply get entertained. So wouldn't it make sense to get in front of these eye balls so you can finally get results in your business?
Starting a business is one thing but growing it is another.
How do successful companies like Dropbox and Groupon gain lots of customers to the point where they don't even need to market any more?
I know it sounds quite extreme and quite far fetched to even think your business could be on par with them, but there are certain marketing techniques they and other successful companies use to grow their business.