I've just created a two-way connection between Posterous and Facebook. Not only will this post echo from Posterous to my page on Facebook, but if anybody comments on it, comments should be echoed back to my Posterous site. Which is kind of nifty.
For this magic to happen, i needed to create a custom domain (for reasons i don't know -- i would happily have used my regular Posterous URL) and a Facebook app. As a Learning Experience, that was okay, but i do fear that you'll need to accept yet another stupid Facebook application to be able to comment.
Anway, first commenter from Facebook gets grand kudos and ten brownie points!