I'm the developer of a few writing apps, and I've tried this approach, you get shut down really quick if you create an account and advertise your app on most forums. I got banned on a lot of them right out of the gate.
Instead try searching for reddit threads on r/writing looking for writing apps, and reply there.
You can also make a twitter account and follow all the thousands of nanowrimo groups and writers groups on there. Most will follow you back and you'll get some traffic
I haven't bothered with Facebook yet (I suck at self-promo) but I'm sure its similar
Find out where these people hang out including hashtags etc. Huge community of starter writers.