Are you creating a v21 viewer? If so, please show a screen shot.
Please try deleting Viewer Builder from the Applications folder and then downloading and installing it again. You should see all four social sharing options in Viewer Builder. If that doesn't work, try deleting and re-creating the viewer in Viewer Builder.
Great, reinstalling Viewer Builder solved the problem!
Now all the four social sharing options have appeared.
Thank you very much Bob,