Which platform do you prefer to start community?

Definitely slack.

Facebook

May I offer a counter view? I've been building communities for over 15 years and have learned something about communities. I'll say it: Please do NOT build your community on Slack, Discord or Facebook. Instead, build an open community. It's 10x better than building a community on Slack or Discord or even FB. 1. Slack and Discord: These seem to be the obvious choice because people already have the apps installed on their computers and mobiles. But guess what; there are problems that no one realises before giving up. It's extremely difficult to keep user engagement on Slack and Discord. Having your community as another tab has advantages and disadvantages as well. Unless your community creates daily/weekly FOMO for users; they won't come back. I've been a part of several slack and discord paid communities; and almost every community is dead or is about to die. Slack and Discord are GOOD for team chats where there's a common goal or tasks need to be done. Plus, you will lose out on SEO advantage that open communities offer. Make your community a user attraction machine automatically without any extra efforts. User generated content is powerful way to attract new users through searches. 2. Facebook Groups These are good to some extent; but you're at the mercy of Facebook. You are also limited to the standard feature set that FB offers to every group. Again, FB groups will send traffic to FB; not to your domain. Imagine ProductHunt being a slack/discord community. All important discussions would be lost unless I'm visiting it daily. Build your community on multiple fronts: Engage your users via blogs, webinars, quizzes, changelogs, newsletter etc. That's how you engage with users. I hope you will find this useful.

Discord

Discord is my favorite

It's Facebook for me! You can know more about people through their profile