A little while ago, I built this:
http://www.littlebray.co.uk/photos.aspx
It's kind of an off-Facebook partial replica of a Facebook photo album gallery, the photos in the gallery section are all pulled from Facebook albums on the Page for Little Bray (fb.com/littlebray) enlarging them gives you a kinda full screen view of them with commenting facilties on the right margin, just like Facebook.
You'll notice that, whilst this comment section looks a bit like a Facebook plugin, it isn't, and the reason for this is that if I used Facebooks Comments Plugin, when a user comments on a photo on the website, or comments on the same photo (remembering the photos come from the same source), it will create two different streams of comments, one shown on FB and one shown on the website.
With my method, any comments posted either on the website or FB will appear in both places as they're al directly injected into the FB graph relating to the object ID of the photo.
A bit later on, I created another, similar image gallery but used Facebooks Social Plugin "Comments" in the right hand margin to comment on photos. The difference here though was that these photos were lcoally hosted by the website and not already Facebook album objects, so this other website is the only source for these images and thus creating FB objects when commenting via a URL is fine.
BUT... it got me thinking, is my first example just a lot of work, is it possible to use Facebooks own Social Plugin to post comments on an object which already exists on Facebook in that way?
I tried pasting a graph URL https://graph.facebook.com/FB_OBJECT_ID into the Comments example on Facebooks Social Plugins page from a status update of mine, but nothing showed.
Any ideas?
EDIT ---
I have discovered I am not alone in asking this question:
Using social plugins for pre-existing Facebook posts
Load comments from open graph object into Facebook comments social plugin
Neither of those have satisfactory answers though. Perhaps I just need to improve my own code and make my own plugin.