Why is it necessary to manually enter a message for Facebook in the mobile app?
Facebook has a few limitations around sharing for third party apps such as Buffer. Because of this, we aren’t able to auto-fill the text update for a post on your behalf.
When sharing to the Buffer mobile apps, there will be a separate composer box that comes up when another social network is selected in addition to Facebook. In this case, the bottom of the screen will say Next.
The first screen that you are brought to is the Composer for all social networks selected except Facebook.
After selecting Next we bring you to the Composer screen for Facebook: