Problem is, when you hit 1 billion users, you've run out of "exponential growth" headroom. All you can do is multiply that by 6.5ish (therefore, linear growth), and even finding new sources of revenue doesn't get you exponential anytime soon.
Certainly, most future growth will come from extracting more cash from the pool of users (or people selling to them), not growing the pool.
Certainly, most future growth will come from extracting more cash from the pool of users (or people selling to them), not growing the pool.