The granularity with which you can recognize trends and speak to exactly what your customers are looking for in copy is far enhanced by keeping a document like this than memory or mnemonics.

I guess I mis-read the post because it sounded like the most important document of my life, not for product management. I will agree that it's important when you're a business and you want to keep tabs on your customers and their use of the product but I still doubt the point that it's the most important document (except for product management or customer development or customer research).

This is an interesting point. I tend to document Q/As that take me more than a couple of seconds to answer. If I logged every time I looked up a question in my notes file, would I get a sense of cycles or trends?

When Richard Branson was asked how to be more successful in business his response was - "Work out"

Aside from it being a great product, they have a ton of viral mechanics built in. The fact that you can't sign in unless you have a facebook friend on the service resulted in people broadcasting on across social platforms which generated lots of buzz. They also prominently display a ton of sharing functionality and let's be honest - people like sharing what music they're listening to