I created https://CoWriteStory.com/ last year, it is a platform for people to cooperatively write stories. I was sending lots of private messages to r/WritingPrompts/ users, and got tons of positive feedback but reddit blocked my account and keep blocking my new accounts. it is a good product but like lots of developers I failed at marketing and attracting users!
Have you heard of http://nanowrimo.org Find out where these people hang out including hashtags etc. Huge community of starter writers.
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