Could Facebook have scaled using MSSQL instead of MySQL?

The issue of scaling has been discussed recently at work and particularly in light of some of the recent details that have come out from Facebook on their data centers.

The question I want to know is do you think Facebook have scaled having been built on MSSQL as compared to MySQL? Did access to the source play an important role? Should it still be called MySQL or has FB changed it sufficiently that it’s really no longer MySQL?