Entry Security - Default privacy of new entries: Public
Facebook - Prompt me to share new public entries on Facebook: selected (but seems irrelevant)
Be logged in to Facebook at the same time, which means this only works if you post from a web browser (i.e. not a client app)
The post must be Public when it's created - changing it afterwards won't make it post to Facebook (If it's Public and then you change it to Private/Friends-only then there will still be an entry in your mini-feed, but you can delete that too - hit the "x" after the entry)
Facebook only shows the Subject (and links to the post) so remember to include one!