Use Reddit! Create a public/private subreddit (whatever suits you). It's a perfect place for discussions, and you can use IFTTT to provide you with webhooks.
I used to have a community that was on facebook before. What a terrible mistake it was. Besides many bad things, it is what you described. There was no way to search for anything.
Reddit isn't perfect, but it's much less intrusive than facebook and people can search for past discussions. I know it is a low bar, but then I don't have to worry about spams, registrations, etc. Plus, if I don't have time to read/deal with it today, the discussion and the person isn't gone tomorrow which chat services tend to do.
