You have mentioned that keyboard responds slowly on facebook only, all other places, keyboard response is good. You have not mentioned the browser that you are using. I will suggest you few steps here:
(a) Use the online keyboard from Start>accessories>ease of access>on-screen keyboard… type from here and see if the response is slow from here as well.
(b) Try another browser like Opera, Firefox, Chrome etc.
(c) See if this happens at all times of the day.
(d) Load facebook with no addons/addins feature of your browser and see if any difference is there.
(e) If you have a multimedia keyboard, install the drivers that came with it
Here’s to Good Karma