Evernote primarily spread through word of mouth:

The company took 446 days to reach the first million users and leapfrogged from 6 to 8 million in just 81 days! Of all the platforms, Android is driving the maximum growth (+1570%).

The total number of premium users has gone up from 35,310 at the beginning of 2010 to over 305,000 in early 2011.

Since early 2011, Evernote has tripled its user base and paying customer base.

Remember, growth hackers: The first million users are the hardest.