Over the last couple of weeks we've been pushing hard to develop awareness for a worthy cause (special needs education tools). Details here:
http://www.tommyteaches.com/special-education.php
(Yes, taking the opportunity to also get a little attention on HN. Sorry, this is important.)
Analytics tells us that LinkedIn, in particular didn't have good engagement. A few strategic posts on CraigsList probably did ten times better LinkedIn.
The best results we got have been due to friends, family, neighbors and local teachers making a push to reach to their immediate connections (their friends, family and co-workers) to have them engage. Also some blogs produced results here and there.
In general terms building an audience for just about anything on the internet is really hard these days. Luck can be a factor, but more and more it seems to require a lot of hard work and "street smarts". For example, I contacted every host at a local TV station with slightly different versions of the same story in order to see if anyone would be interested. Call it A/B testing. One was, and that helped a bit (just a bit).
Good luck with your app. It looks interesting.