In another sign of the explosive growth of Twitter Lady Gaga has become the first tweep to have over 10,000,000 followers. Even assuming a 10% congruency that means Ms Gaga can reach 100,000 in real time at any time.
Now Facebook can make a great deal about its 600,000,000 users, but it lacks the ability to broadcast to the kind of mass audience that twitter has. Facebook is more person to person contact, though it is evolving more and more in to a social enabled news aggregater. But twitter has the raw broadcast ability.
BBC - Newsbeat - Lady Gaga is first celebrity with 10m Twitter followers