Ask HN: How have you hacked user acquisition using a mobile app?

Clearly, one of the first things to think about is mobile use, I get that - if noone is visiting on mobile, why make a mobile friendly version.

What I'm getting at really, is, are there reasons that you have decided to make a mobile app to interact with your site, that had ulterior motives, what sneaky things have you done that made users feel more.. empowered, special, exclusive, or maybe it only exposed things that you can do quickly, responding to messages, posting new items..?

I feel like exposing aspects of any site can lure in a potential users, users that one has not thought of in previous discussions..

So HN, how have you hacked your user acquisition by providing something simple, something mobile, that seems to work suprisingly well?