There are a few different ways to share an activity to Facebook. One is manually on a per-activity basis by selecting the share icon from either the mobile app or website. The other is a setting on the save activity view that can be turned on to automatically share all new activities to Facebook as soon as they are uploaded.
How automatic Facebook sharing works:
Directly after uploading an activity, you have the option to edit the activity before clicking save. You can share the activity to Facebook by flipping the Facebook toggle ON from this view. However, this toggle is a default setting that will automatically share future activities unless it is turned off.
To turn off the automatic Facebook sharing:
Toggle the Facebook share option OFF on the save activity view on your next upload.