How to disable “Seen by” in Facebook chat?

The only thing you need to do is to block the url

facebook.com/ajax/mercury/change_read_status.php

For Chrome, I created the extension FB unseen which blocks it, but you can use any adblocker to achieve this. Additionally, I added a "Mark as read" button, so instead of removing the feature completely, you can decide when the "Seen" flag should be set. Be aware that opening the "full conversation" page marks it as read, too, without using the change_read_status.php script!

Edit: The thing about the "full conversation" site marking your conversation as read is not true anymore since facebook rolled out the new messenger. (Also, my button marking it as read is therefore not working anymore, too, working on a fix)

You cannot. There is no option to turn it off with Facebook UI.

Workarounds

Theoretically, you could kill all internet connections on clicking the message icon and prevent Facebook from using an event listener to check when you opened the message.

Going by this assumption, I'm saying that the read receipt shouldn't fire until you actually open a message as you are not able to read all of it. Still going on assumption, if Facebook preloads the message before sending a receipt and before you kill the data you should be able to inspect it. Then you would have to look into the timing of the message.

Another way would be if you do not have a window open with Facebook allow the notification to go to your email and reply from there.

A third way would be to access your data via the API using the Graph API explorer http://developers.facebook.com/tools/explorer/?method=GET&path=me%2Finbox which would honestly be a pain.

All in all it would just be easier to open the message and let the read receipt send. BlackBerry users (such as myself) have had to deal with this for years :S

I just created a simple facebook application that uses the Graph API to access your messages and shows them in a reasonable way. This way you can always check your messages without "being seen" and without having to jump through hoops like the above. Well, other than using the app.

https://apps.facebook.com/unseenly/

In AdBlock Plus or a similar extension that can use ABP filters, you can add facebook.com/ajax/mercury/change_read_status.php$xmlhttprequest as a custom filter, which will disable the other party from receiving the 'seen by' notification.

The Opera extension, SocialReviver has a feature that disables the 'seen by' notification.

Its features (emphasis mine), from its Opera add-ons webpage:

Features:

• Disable the new Profiles and Pages layout ("Timeline") and return to the Classic 2011 Profiles.
• Disable the Chat Sidebar and return to the Old Normal Chat.
• Use Friend Lists in Chat to limit your availability and group your Chat contacts.
• Don't let your friends know when you read the messages they send to you in chat (experimental feature).
• Split aggregated game posts into single posts. (NEW!)
• Link the name in chat windows titlebar to contact's profile.
• Revert the blue top bar to the old style one.
• Stick the blue top bar at the top of the window.
• Add a shadow below the blue top bar.
• Enable or disable extension features via a settings panel.
• And many other small but useful features!

you can simply use this Chrome Extension to disable any read status connections: https://chrome.google.com/webstore/detail/gfpgaanechfneiboempkfjghninbibjn

Preventing the cause implies preventing the problem. A person has “seen” your message when:

• For the messages window— when they have clicked on your message from the messages window. Prevention: don't click on their message. Just view its preview.

• For the chat window— when the user, in any way, has interacted with the chat box. You can, fortunately, know if a person has the “seen” message. When you receive a new message and it opens a new chat box, you can see a blinking title bar [of the chat box]. If you click on the chat box in any way, the blinking stops. So, blinking implies... not yet “seen”. Prevention: just look at the message but down click the chat box. That includes closing, minimizing, just clicking et cetera.

I just made this and it doesn't rely on any browser extension - Facebook Ghost Messenger allows you to read messages without notifying other participants.