The SEO Facebook Plugin is awesome and I use this on my blogs, it boosts the SEO power with Facebook comment system, however the "Hide old comments" will be taken as spam by the Google Search Engine. As Matt Cutts stated in a post, the texts, which is hidden from real people and shown only to crawlers will be treated as spam and Google will penalize your blog.
There is a way, Old comments should be visible, but there should not be any "add comments", "post comments" option effective from the day, since we setup Facebook Plugin.
I mean: Old comments should be visible, but there should not be any options to post comments expect the Facebook one. I hope the Author of the plugin will help on this.