You could easily change the tone here. "I joined a bunch of very specific types of groups so I could manually write a post that tells them about a free app that I built for their demographic."

I felt the "scum" right away, but then I thought about the source of the feeling. Scummy is when you write bait. Telling people about your app - sometimes that requires a door to door cold call.

I think this app deserved to be shared with that group, and it's particularly un-scummy considering the fact that it's brand new, free, and isn't trying to mine for email addys.