First on the list, is Instagram. I am not very familiar with Instagram but was impressed when I discovered a whole page of pictures and links about Downtown Abbey, which you can find here.
From Instagram, I found the official Downton Abbey page on Facebook. If features more great information and pictures as well as bonuses like a quiz to determine which character you would be in the series. You can find the Downton Facebook page here. I th ink that the Facebook page will be a great source of information about future developments in the world of the Crawleys.