There are two "modes" of Facebook sharing on Strava: One is on a per-activity basis, and the other automatically shares all activities to Facebook as soon as the activity is synced.
How automatic Facebook sharing works:
Directly after uploading an activity, you will see the activity edit page before clicking "Save & View." You can share activities to Facebook by flipping ON the Facebook toggle for any activities you wish to share (button turns blue). Keep in mind that this toggle will turn on automatic sharing. In other words, all future activities will be shared as long as this button is left on.
To turn off the automatic Facebook sharing:
You can do this by turning "off" the Facebook button on the ride Save screen (looks like the letter "f" next to the lock symbol). By turning this off, you are disabling automatic sharing to Facebook (button turns grey).
Then, in the future, you can share activities individually. After completing the details on the save screen, simply save the activity and let the activity sync in the Feed. Once synced, click on the activity to view the details and then click the "Share" button in the upper right corner. Select Facebook.