For Facebook to interpret an image as a 360-degree panorama, it needs to have the following properties :
- It must be in 2:1 aspect ratio. eg, 4000x2000 pixels
- It should be in JPG format (it seems that the panorama settings are ignored for PNGs!)
- It needs the EXIF tag 'ProjectionType' set to 'equirectangular'.
For more information see this Facebook link about Editing 360 Photos & Injecting Metadata.
To achieve this, once you have rendered your image (using an Equirectangular camera - which should be oriented level with the horizon rather than tilted) and saved it in JPG format, use a tool such as 'exiftool' (or another tool that allows you to manually specify EXIF tags) to set ProjectionType - eg, with the following command line :
exiftool -ProjectionType="equirectangular" <filename>.jpg
This will add the relevant EXIF tag to the image (saving the original as an '_old' file). The resultant file should now be successfully interpreted by Facebook as a panoramic image.

EDIT : Here's an additional source of useful information - http://atterer.org/tech/android-exif-tags-xmp-pano-panorama-exiftool, including additional tags that should allow you to tweak the panoramic view as desired (NOTE : I don't know which of these are actually honored by Facebook, but worth a try) - replace the settings with your own image dimensions, etc. :
exiftool -UsePanoramaViewer=True -ProjectionType=equirectangular PoseHeadingDegrees=180.0 -CroppedAreaLeftPixels=0 -FullPanoWidthPixels=8000 -CroppedAreaImageHeightPixels=4000 -FullPanoHeightPixels=4000 -CroppedAreaImageWidthPixels=8000 -CroppedAreaTopPixels=0 -LargestValidInteriorRectLeft=0 -LargestValidInteriorRectTop=0 -LargestValidInteriorRectWidth=8000 -LargestValidInteriorRectHeight=4000 filename.jpeg