Okay I deactivated the WP Social SEO Booster plugin, cleared all my site caches, then added a new post. It still did not publish to Facebook. This is not too surprising to me, since WP Social SEO Booster and Jetpack had been living together in peace for quite some time on my blog when everything was working.
Again, the only things I changed between when it was working (publishing to Facebook) and not working was updating Jetpack to version 2.7 and updating Wordpress to version 3.8.