I am confused because weblogs.com shoots back a friendly "thanks for the ping!" bit of XML after a successful ping.
I can't seem to find any documentation on pingomatic API anywhere.
Matt writes that he created one 6 years ago, but there are no links on this page: http://blog.pingomatic.com/2006/01/16/internal-api-complete/
EDIT: I think it might be down to anything but pings from wordpress blogs, but I don't understand how they're doing that.
It doesn't. You can no longer ping pingomatic via API. No idea how it allows facebook blogs to do so but oh well!?