If your share buttons on your app are not working properly, it could be that you edited your share links improperly. You normally don't need to edit your share links at all... they just work as is. Here is how to solve the issue by reloading the default share buttons.
Delete the current Share Buttons section in the layout sections portion of the builder by clicking on the trash can next to it. Then press the add section button and select the Share Buttons section. This will add a new default Share Buttons section at the bottom of your app. Drag it to the correct position. Publish your app and you should be good -- for this app. You may need to repeat these for your previous apps if they are not correct.
Your default layout may be not correct. To fix it, Create a new app, repeat the above and then click the Save Layout button. That should have you setup for your future apps. You can delete this new app you created as it is no longer needed.