They do not disclose.
It would probably be more accurate to call them "users" but some of them are paying. That total is total users which encompases both ad supported and premium users.
I am pretty sure what you see is what you get. This is their model, they have implemented it and taken it from losing money to making money. The only thing they "havent found yet" and I assume will continue to evolve is their ability to deliver different types of targeted ads to their ad supported users. They just got a big injection of VC funding in a bad economy so, others are seemingly on board with the model.
Like I said, I am not concerend with their ability to make billions (although that is even scarier), what I am concerend with is them even breaking even. Breaking even means that they will continue plug along and strangle this piece of the market.