If you use Adobe Photoshop Lightroom in your photography workflow, you're probably publishing photos to Facebook in some fashion. Here's an awesome post on how to create custom Facebook Cover Photo collages right in Lightroom's Print module: Lightroom 4 Print Templates: Facebook Cover Photo Layouts | 27ShutterClicks Blog.
There are a handful of useful templates:
And here's a sample I threw together:
My recommendation: put the photos you want to choose from in your Quick Collection to easily add them to the layout. I'll definitely be using this tip often; enjoy!