From the article: "The firm did this by tapping into a 63 million share over-allotment option, or greenshoe, according to sources familiar with the deal."

The title of your post is inaccurate. No one knows for sure how many shares MS bought during the initial day of trading.

For a full breakdown of the first day of trading, check out Zerohedge's analysis:

http://www.zerohedge.com/news/facebook-complete-forensic-pos...