Great for a small community
Slack's user experience is great. It's really easy to use and the chat style is very conducive to engagement. If you're wanting a conversational type community, this is an affordable option. One thing to note: if your community is really big, I wouldn't recommend Slack because I suspect the chats will move too quickly to dive into specific topics. They have a free version, so I'd recommend signing up and giving it a go if you're considering it as an option!