When two or more people chat together, Seen by appears when one of the participants views the last message.
It is nice to know your message has been viewed, but it is also an invasion of privacy.
How to disable Seen by?
(using the desktop web interface, not mobile)
The only thing you need to do is block the URL:
facebook.com/ajax/mercury/change_read_status.php
For Chrome, use the extension "FB unseen" which blocks the URL (full disclosure: I wrote this extension). If you want you can use any adblocker to achieve this. But with "FB unseen" you can "Mark as read" whatever you want, so you can decide for yourself when the "Seen" flag is set.
One drawback of this extension is that all your messages will be marked as unread for yourself, too, and the "unread messages" notification stays visible as long as you do not reply (or explicitly mark it as read).
Update: Facebook requested me to take the extension down since it violates their Terms of Services. Since it was developed open source and the source is available at GitHub, I updated the link to point to the GitHub Page. To not risk any more trouble with Facebook, I will not provide an instruction on how to install extensions manually and will also not continue developing it. The extension was previously available at https://chrome.google.com/webstore/detail/ihcedcpmfdpjijiamkaeaefgfagnnpei, you can find a couple of alternatives in the store by searching for them.
In uBlock Origin, AdBlock Plus, or a similar extension that can use ABP filters, simply add the ff. as a custom filter, which will disable the other party from receiving the seen by notification:
facebook.com/ajax/mercury/change_read_status.php$xmlhttprequest
If you'd rather not use any of the above browser extensions, you could also just archive the message from your message inbox (by pressing the 'x' button on the left pane), sending it to the message archive. The message will remain unread, sending no 'seen by' notification to the other party, and will now be gone from your inbox (you can still view it later if you chose to at the message archive).
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
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.
Another Chrome extension: Facebook™ Chat Privacy
Facebook Read Status Disabler automatically blocks "seen" + "is typing" status confirmations in Facebook Chats and Messages.
The extension has overwhelmingly positive reviews at the Chrome web store.
I've created a simple Facebook application that uses the Graph API to access your messages and display them in an intuitive way: Unseenly
With Unseenly you can always check your messages without "being seen" and without having to install a browser extension.
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.
I've written http://www.inboxn.com/facebook-unseen#whatis
Reading your Facebook message with it does not mark them as seen.
Being a separate website, you can still go on Facebook and set your messages as read/seen. This allows you a greater control whether or not your friends can see that you have read their messages.
