Metro style is all about the gravity of importance to the user, in this case the owner of the website.
Keeping that in mind the plugin should be flexible enough so that if a particular social media format is preferred the squares should reflect that importance.
Facebook is a fading social platform as others are growing, such as Google+ for business integration, Pinterest is very high in female traffic. In these examples, it makes more sense to have the web site owner direct visitors towards the social media of choice by allowing the main box to be that choice, i.e. the largest box.