A community is a must-have if you want to build a successful social business. Let's quickly analyze the available tools to build communities:
Slack - a messenger app. Pros. It is familiar, widely used (for work). Cons. Have limitations with the free version (chat history), they definitely need to improve the search experience, and personally, I don't like having another (heavy) app on my devices;
Discord - a free messenger app. No limits as of Slack. Cons. IMO it has so many features which makes it hard to adopt.
Telegram/Whatsapp - Pros. easy to join, easy to invite members. As they are used for other purposed, they are available on all devices, no need to install. Cons. Group management is not advanced. When the group gets large it is hard to manage both the users and content. No subgroups, everything is in one space. The member-to-member conversation could be challenging. As a starter, these tools could be a good option.
[Circle.so](http://circle.so) (and other web-based tools). Pros. You don't have to install an app, it is all in the browser. There is a number of features such as email sending, content security, payments. You own users' data. Communication is asynchronous. Cons. It is paid, it is required to do some configuration customizations.
Of course, you can build communities on Reddit, Twitter, Facebook etc. But you should be aware that with the social networks you do not own your data, there are strict rules, by violating which your group could be shut down.
There are some new players in the field. e.g. the recent one is [https://geneva.com/](https://gen..., all in one app to build a community.