Hopefully a step by step article is on the way this week.
You need to 1st set up the integration in Admin -> Launchpoint. You have to create a new service, select Facebook,
Click authorize Marketo. Then you will be prompted to login into Facebook and grant Marketo permission.
Then on any smart list you should see the facebook icon at the bottom. Clicking it will send the members of your list to your custom audience in Facebook and Facebook will search for a match. (But you won't know who is or isn't a match)