OK so let's start with the good. This plugin, hands down, is probably one of the best plugins we've ever used. On our main (home) site the plugin works almost without flaw (aside from double importing comments from time to time). You can see how we use it here.
Notice we're using a hybrid of Facebook comments using the hook, then below that we use regular WP comments. Most of those comments were imported from our Facebook page (16,000 fans). You can tell this from the front end by hovering over their name and you'll see their FB URL.
Now, where the plugin stops working on the import side is on sub-domains.
No comments, yet that page had 2,400 visits from FB. The link was manually posted on multiple fan pages that we run, all of which are instructed in the settings to download from.
One of these posts can be seen here.
The settings in the backend of this sub-site are almost exactly the same for the main site. In fact, the only difference is that I have not yet coded in the hook for the Facebook comment box on the sub-sites.
Now, at first I thought it could be that we're running mapped domains on the sub-domains. I then tried posting the actual sub-domain to the FB page, but it still would not download those comments.
I would use something liek this, for example.
No go. No comment downloading.
Another sub-site with this problem.
Comments won't download.
Everything else works. Cumulative "likes" work, site avatar on FB posts works, etc. Just not the comment downloads.
BTW we're running both MS and BP.