I checked with Denise (one of the owners of Dreamwidth) to find out if she knew about it, and discovered that Facebook have stuck Dreamwidth on a block list.
With no explanation as to why, no way to contact them, and no appeal.
This is unbelievably frustrating. And the kind of centralised, autocratic, opaque decision making which I loathe. Tens of thousands of active users, unable to share blog posts with Facebook (which, let's face it, is where most of my friends go for their socialising).
If anyone knows anyone who works at Facebook, do see if you can prod them into getting an explanation...
Edit: All working fine now.
Original post on Dreamwidth - there are comments there.