What does 'Done' do?
The 'Done' option helps you filter all inactive conversations under a different category.
If you have some messages/posts/comments you do not wish to see in your inbox, just mark them as 'Done'.
These messages don't get deleted. You can view them anytime by clicking on 'Done' from the Inbox drop-down menu. You can also move them back into your inbox by clicking on 'Move To Inbox'.
Note: 'Mark as Done' option helps you remove all the inactive conversations (posts/comments/messages) from your inbox and focus on the active conversations. This option applies only to your Social Inbox and hence, the changes don't reflect on your Facebook Page.