Wow, well... maybe it's just my ignorance, these people do this for a living. Just seems like a crazy number to me. I'm sure there's a lot of detail and reasoning behind that number.
Running a successful startup is crazy and unnatural; that's why it's so rare and that's also why there's backlash against that becoming the default mode of thinking.
Based on a quick calculation, to get from zero to one (billion) users takes just under six years at 7%/week growth.
Based on a quick calculation, to get from zero to one (billion) users takes just under six years at 7%/week growth.