Facebook added "Seen On" feature in chat quite earlier and it seems most of the users didn't like the new feature. In Facebook Chat, when you send a message to your friend, the Facebook chat will display a "Seen On" time on your Facebook chat box once your friend sees your last message.
Just like this, the chat box also displays "Typing..." at the bottom when the friend you're chatting with types.
This annoys me a lot and I think most of you too. At least Facebook should have given an option to disable it officially for those who don't want this feature. And I am sure, if Facebook gives the option to disable "Seen" and "Typing" feature in future, I will never be seeing it while chatting with anybody as I know most of the users will disable it for sure.
Though Facebook not officially allows it, but there are a lot of workarounds to get rid of these features. The simplest method is to block the following URL, because this URL is responsible for those irritating features.
facebook.com/ajax/mercury/change_read_status.php$xmlhttprequest
You can block the above URL using the Adblock extension for your browser, which is available for major browsers.
The second way is for Chrome users, is to use Facebook Chat Pro extension which you can download here. The extension gives you separate options to disable the Seen and Typing feature. So, you can leave any of them active while disabling the other if you want.
Along with this, the extension enables Chrome desktop notifications and displays all your Facebook notifications on your desktop.
If you're a Firefox user, you can use Unseen extension which does the same thing. But it doesn't allow you to disable the "Typing" feature.
One simple tip for Mac users is to avoid chatting with Facebook friends using the Facebook's chat box, if you don't want to use any extension to disable the Seen feature, you can use OS X's Messages app to chat with your friends. I use the app for chatting and I found that Messages app doesn't supports the Seen feature.