It'd be difficult to choose one platform that serves the needs of every community. The choice of community platform depends on the type of your community: Open Vs. Closed Community
I am an advocate of open communities because they are easier to grow.
That said, you have multiple choice. Here's how you can decide by answering these questions:
- Do you want to grow organically?
- Are you okay exposing your content to people without registrations?
- Are you okay with having users discover your community through search engines?
If your answer is yes - you can choose community platform like Jatra. There are others like Discourse, xenForo, Flarum, phpBB, vBulletin, Mighty Networks, Better Mode, Circle. Each platform has its pros and cons; and price associated with it. For example, Jatra is completely managed community platform and takes care of hosting, security, updates, bugfixes and feature releases for you.
Some open source community platforms like Discourse, phpBB and Flarum will not require upfront payments. However, you will be responsible for managing them and the servers.
If you answered no - then you can go with Discourse or Slack. The free versions have limitations - so be aware of those.
Facebok Groups and LinkedIn are also good - if you wish to leverage your networks. The problem with these communities is that the community is never under your control and you are limited by the rules and algorithms of the hosting networks.
I hope this helps. I'll be happy to help you decide, if you share more about your community requirements.