Suggestion: Private discussion forums for teachers/classes
This is what my friend and I implemented on our own private website. We used SMF forum software - through cPanel. The pros: You get to set up the forum your way. And, it is possible to ban registration and instead register your students yourself. Then, you send them their username and password details. I forget how to do this now. Also, you can have boards inside of boards inside of boards, if you want. You can have a splash page that requires username and password in order to even get to the forum, if you want to make it harder for crackers to even find the forum. Its not fool proof, but they'll go to a easier site usually.
Cons: Unless you set up the forum properly such that 99% of the content can't be seen except by members, the forum may be public. OTOH, anything your students post on Duolingo's forum is technically public. Anyone can join and see.
My friend and I love our private forum. It just works for us. We can communicate in whatever language we are learning. We can post questions to each other and find those topics faster. And its not one page with tons of topics. Instead, we can compartmentalize it as much or as little as we want.