There's a lot of good information the slides, but for the quick and dirty:

1. They found one feature that a lot of people enjoyed (some would call this a MVP) and lots of people shared it

2. Mashable picked up on it and then other publications did.

3. Massive growth.

The main takeaway that I always tell startup people:

1. Marketing works, don't neglect it. (in this case PR worked extremely well)

2. Don't do marketing until you've found the one feature everyone wants (i.e. product-market fit)