I ran a beta period for one of my apps which I consider really successful, lots of feedback and lots of activity. My way to go was to seek out people commentin

I ran a beta period for one of my apps which I consider really successful, lots of feedback and lots of activity. My way to go was to seek out people commenting positively about another product, then I would send them a message to see if they were interested in testing. This guaranteed that I would get, dare I say it, passionate people to test it - and I did. One thing to note though was that I was looking for comments about "productX" which was already mine, and the new product was essentially a "productX 2". But I guess it can work somewhat similarly within a genre or category. Maybe not as perfectly matched as for me.