I've been using Piazza in my large (n=+- 1000) course each semester since about 2013 and it is one of the teaching tools I love most.
Piazza describes itself as:
I have 1000 students asking questions, and often the same questions. With Piazza my students no longer email me these questions. Instead they post them to Piazza and then their fellow classmates have an opportunity to answer them. Normally the questions get answered pretty quickly but if someone asks a question that the class doesn't know the answer to, then I can jump in and clarify things. An instructor can also endorse "good" student answers thereby giving students reassurance that their classmates have got it right.
Piazza also provides instructors with handy statistics from their class. Looking at these I can see that I get about 1300 questions a semester and about 90% of those are answered within 20 minutes by a student in the class. I can also see which students are asking the most questions, being the most helpful by answering questions etc.
Piazza is free for instructors. They make their money by advertising jobs for students. The adverts are aimed primarily at the American market so we Australasians tend to get the service for "free".