If you create the logo with tranparency the map will show through the logo. I created a test logo that will go out with the next beta. Here is what it looks like this:
I used Paint.Net to create the image and make the background semi-transparent.
Note: After discussing the facebook implementation with one user, I discovered that you can only post to your personal page. If you manage multiple facebook "pages", the current implementation will not post to those pages. I have been testing changes that will allow you to post to any page that you administer. Those will be available in the next beta release.Update: Beta 8 now includes support for multiple facebook pages.