Lady Gaga was the first Twitter use to reach 10m followers

"Hurrah there is a winner and it is Lady Gaga. The queen of pop has become the first Twitter user (really superstar user) to soar past the 10 million follower mark. She beat Canadian popster Justin Bieber to take the 10 million crown.
Lady Gaga passed the milestone yesterday and has since gone on to add another 50,000 followers to give her a grand and rising total of 10,052,574 compared to Bieber’s 9,662,626 followers."

Source:  The Wall blog, 16th May 2011