This box (shown below) is the flash share message. It is loaded from the template _flash_share.html. You can edit it there.

The 'Thank you' message at the top is loaded from the {{ flash.message }} variable. It cannot be edited, as it changes for each page type. 

You can edit the 'Tell all your friends...' message in the _flash_share.html file. This will change for all pages.

The default share message can be changed in the page settings under Settings > Social media > Default post.