Social media - I am a web guy, fell in love with Internet and professional webmasters. I really enjoyed the RSS/blog period, then Social media came and aggregated everything. They close RSS and readers like Google Readers to force people to go to their sites. Before, readers could go directly to the authors in an self-hosted website; now everything is posted on social media... content is no longer ours. Social media killed the web. I am in social media but just to develop code to automate things (API, code that get content, automatic repost). I have several Fb pages which run alone, it goes pick images there and there and repost
There is not better than old-school forum in my opinion
Private / Public post - I shall share what Okie said, not sure if putting everything in public is fine. If you do, you should indeed allow user to request for specific post to be removed... Or create a new user and ask admin to delete all posts under the old usernames.
Get more people -
a) Rework the landing page to have more attractive public forums which catchy names
b) Have a giveaway section and try to organise a monthly giveaway or things like blind Santa-chain (names go in a pot, then you send a present to the name yo are given, sorry no idea how it is called)
c) Spread the love on social media; meaning automating things is the way to go... you just need to pinpoint the perfect repository or website (this forum, an Instagram, a Google Drive, Blogpost, etc.); register APF in Twitter, IG, etc. and finally work on automation so that anything posted there is automatically reshared on Twitter, IG, FB, etc.
d) Ask members to contribute more - just like in some threads here ("poster in movies") for a more participative website; may have more participative threads (I recall I did the "shot between the two legs" threads)... a nice would be "scissors in movie posters"