In order to set up automatic trip publishing, sign onto TripIt using your Facebook log in and password, then click the link in the upper right hand corner of your profile page that says "Find friends and share trips with Facebook Connect" and follow the steps. It only takes a second or two.
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So, how do you know if you've linked up? I've got it in my facebook apps list, but I still have a "link up to facebook!" link on all my pages, and I can't seem to turn on auto-posting of notification to facebook in my account settings..
Now all we need is to be able to find our friends on TripIt using our Facebook accounts, and the circle of life would be complete :)