I will soon start work on a side-project that involves building a app to allow users to share messages about a specific "vertical" with each other. ( I know this is a pretty vague description - I plan to present it to hacker news after getting it to beta stage).

Challenge: How do I make the app popular? Specifically, how do I get the initial adopters? I believe that once we get critical mass and power users on board, then network effect will take care of ensuring that more customers come on-board.

But how to get the early adopters? Any suggestions?

There doesn't seem to be a clear answer about how instagram accomplished it: http://www.quora.com/How-did-Instagram-get-so-many-users-so-quickly http://www.quora.com/Instagram/How-did-Instagram-get-its-first-25-000-signups

Thanks, Dhar.