When the share icon doesn't work or isn't visible on the website listing details page, it is because of the widget needing an upgrade.
How to know this?
It's easy! Just go to the Website Module > Pages > Single Listing Page > click the pen icon of the listing details widget for the side panel to open > you should see an upgrade button.
Once this is upgraded a new widget will be added on top of the old one and the Share Icon should be working and visible.
Once you have checked and confirmed that the icon now shows, delete the old widget so you are left with the updated one.
Alternatively, this may also be caused by the icon color being set to white. To adjust this, go to Styles > Share > update the icon color to your desired color.