When pasting a URL (http://facebook.stackoverflow.com/questions/###) into chat, I see the "working" indicator image for a second or two, but a onebox is not created.
Editing the post and removing
facebook. from the URL makes it behave normally.
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Works now. Since facebook.so is not a "real" site, it's not in the StackAuth sites listing, that's why the links weren't recognized as "oneboxable". Until we have found a good way to expose mini-sites via the API, facebook.so is now hardcoded as being okay to onebox.
Note that even when you use a facebook.stackoverflow.com link, the link in the onebox will go to the canonical resource on stackoverflow.com.