How Can I Turn Off "Seen By" Messages in Facebook Chat?
More than ever, we live in a read receipt world. Your friends (and enemies) know when you've seen their message. But it doesn't have to be that way in Facebook. There are several extensions that can help you keep your frenemies in the dark. The web apps experts at Stack Exchange offer up some options.
When two or more people chat together, "Seen by" appears when one of the participants views the last message. It nice to know your message has been viewed, but it is also an invasion of privacy. How can I disable "Seen by" so that people do not see "Seen by Nicolas"?
FB Unseen (Answered by swege)
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, with a little work you can use almost 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). Another drawback: there are ads (which you can disable).
Unseenly (Answered by David Verhasselt)
I've created a simple Facebook application that uses the Graph API to access your messages and display them in an intuitive way: Unseenly allows you to check your messages without being seen and without having to install a browser extension.
Facebook Chat Privacy (Answered by Lukas)
Another Chrome extension: Facebook™ Chat Privacy automatically blocks "seen" + "is typing" status confirmations in Facebook Chats and Messages. The extension has overwhelmingly positive reviews at the Chrome web store. :
Adblock Plus & SocialReviver (Answered by galacticninja)
In AdBlock Plusor a similar extension that can use ABP filters, simply 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. Find helpful instructions on how to do this here. Another option is SocialReviver (available for Chrome, Firefox, and Opera), which allows you to customize Facebook in various ways, including disabling "seen by."
Have experience using the services mentioned above? Have your own expertise to contribute? Check out the original post, and see more questions like this at Web Applications Stack Exchange. And of course, feel free to ask your own question about web apps.