1. Manual method: https://www.survivehive.com/adding-social-bookmark-icons-mailpoet/
2. With a Plugin: https://wordpress.org/plugins/mailpoet-sepideman-social-icons/
You can replace the 4 default social bookmark icons for Facebook, Twitter, LinkedIn and Google Plus.
For example, you might want to have Pinterest, Youtube, or Vimeo icon.
This is more of a hack that requires intermediate tech skills. It takes about about 15-30 minutes to complete.
Locate the folder of social bookmarks on your FTP: wp-content/uploads/wysija/bookmarks
Look at one of the numbered folder with the icons in png or jpg inside, like this: wp-content/uploads/wysija/bookmarks/12
Recreate your own set and upload. Your new icons need to be named:
Note: MailPoet won’t recognize other names, like youtube.png!
Your set should be visible in the social bookmarks in the visual editor after you’ve uploaded your new set of icons.