In order to complete the assignment that requires you to share a post to Facebook, you need to fully complete the share process. Previously, the mission was to share OR challenge (invite) a friend. Now the mission is only to share.

In order to share, you need to actually click the share button, according to Facebook. You can't just start a share, you have to actually post it to your feed. 

If you always cancel the share/post before posting, you will no longer receive any credit for completion. 

Please make sure you actually post the share and that you are trying to finish the challenge by sharing, and not by challenging friends.