BTW, I'm the OP and am happy to answer questions here. My expertise is obviously in consumer internet products but I spend a lot of time thinking about growth. It's my superpower!
What's a "very modest spend" when it comes to advertising? How much is enough to validate whether and ad is or isn't working in your experience? Also did a majority of your new users come from being featured on app stores or was it primarily through turning the modest spend into a serious spend once the ad was converting?
Our app is free, so we aren't eager to pay for installs at any price, and paid aq is not a major channel for us. The lion's share of our new users come from organic search, social sharing, and word of mouth.