Seesaw is not ideal for online discussions as it doesn't allow kids to reply to each other—an essential aspect of any discussion (you're still better off using a Google Site/Blog for this). However they can reply to you, and there are times when you don't need a discussion, all you need is every child in the class to post one comment in response to a prompt from you. This is great as a simple 'entrance ticket' or 'exit ticket' or just as an excellent way to help kids to 'tune in' to an area of inquiry.
When you do this, you should probably switch off notifications for students comments, even if only temporarily!
Here's an example from Lorena's Grade 4 class: