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

I've been trying a similar approach. I developed an Electron app called First Novel App which is supposed to help writers silence their inner editors.

It's available free to download at http://www.firstnovelapp.com which redirects to a Gumroad store.

thanks for the tip, will investigate.