Hi Sven
The extension does not support instagram, so if you want it, you will need to modify code. The file you need to modify is components/com_eventbooking/themes/default/event/default_social_buttons.php to add the icon. If you modify the layout to add icon, please move the modified file to template override so that the change won't be lost when you update to future releases of the extension
Regards,
Tuan
