So I've installed Yoast's SEO (which is what I'm assuming you're using to generate the Open Graph tags), and I still can't re-create the problem.
1.The Yoast plugin is providing an og:description tag on all my posts, and Facebook is using it. Your pages don't have og:description tags - probably because of whatever you've done to turn off excerpts.
2. I tried leaving the description blank for my home page in the Yoast settings (to force my home page to not have an og:description tag) and even then I can't re-create the problem (Facebook is grabbing "Mike Toppa's blog" from my masthead div as the description).
It seems to me this is happening because your site isn't fully setup to use Open Graph. You should have an og:description tag (as the Facebook debugger is advising you to do), or a meta description tag.
Without those things, Facebook is scanning through the page content looking for something it thinks might be the page description. I don't know what their algorithm is, but there must be something about the names of the shashin classes that it's latching on to. Without being able to re-create your problem, I would be guessing blindly at renaming my shashin classes (and I'm not using "description" as a name anywhere).
It seems to me that if you want to use Open Graph, you should supply it properly with the fields it's expecting. Otherwise there's really no predicting what it will try to use as the page description.