In your marketplace stall set-up, you can link to a website, facebook page, ebay store, etc
A thumbnail should then display or the web address that you have linked to.
However, if that web page can only be viewed if you are logged into the site, then no thumbnail will appear.
This is the case for Facebook pages. Facebook pages can only be viewed by somebody who is logged onto facebook - so no thumbnail will appear. Instead there will be a camera icon with a red 'stop' hand inside it.
Whilst facebook is great - remember that pages can only be viewed by people with facebook accounts who are logged in. Consider also setting up a free blog with wordpress or google blogger so that you do have a presence that can be seen without logins and passwords.