What happens if I leave the Icon Class empty on a share button?

Icon Class is optional. If you leave it empty, the button shows the first letter of the button Name inside the round icon, for example "T" for Telegram. It still works the same way.

The presets under Quick add use classes in the style fa fa-facebook. If you know the matching class for your network, enter it in that style.

Edit the button in Marketing → Social Sharing at any time. Hovering over a button shows "Share on" plus its name.