I love the way you are thinking.. I will share my experiences on this. Let me address your question first. Yes, You can use map api to find the distance between two lat/lng.
Lat/Lng calculations are very CPU intensive. So during the storage I use a formula to allocate a house for a point. When matching nearby location, I use the house which is a integer in nature with a > and < query rather than calculating distance between two points with loads of math floating point calculations.
I faced problem on generatng content. It's a chick egg issue. If u don't have content people don't come. If people are not there, there is no content. So the really road blocks will be on content generation.