Thanks for contacting us! Yes, this has been developed. To create a Facebook cover collage, simply create a collage as normal, and select “Facebook Cover” from the Snap dropdown (towards the end of the list). This will create a collage in the correct proportions for a Facebook cover photo. The actual dimensions will be larger than 851×315, however Facebook supports larger cover photos as long as they keep the correct proportions.
Let us know if you have any other questions!