Facebook had no direct settings for this, Infact they are very poor at your privacy, they sell your valuable data and get millions of money, but they had given this in their TOS , irony is several people wont read this and never mind this.
How to Disable or Remove Seen Feature in Facebook
1) By using ADD ON'S
- For Google Chrome , download the FB unseen and add it to your Extension list
- When you are going to download the FB Unseen this window will open, add the plugin to your chrome bar and restart the browser
- People will not get Seen on their messages even you had read those messages
For Mozilla Firefox
- First you need to install Grease Monkey script into your Browser to remove seen feature
- Then add Facebook Stealth to the Browser and restart it
- Now your Last seen was disabled and you can happily send messages
For Internet Explorer
- Download the Chat undetected and remove the seen feature in Facebook
If you do it Manually, open the chat box and click on Read full conversation and you will go to main messages box, there Click on Actions and Select " Mark as unread "