At Gamefest 2007, Microsoft made a speech regarding just how successful Xbox Live is. Microsoft's Game Developer Group general manager Chris Satchell says that there are currently 7.1 million paying Xbox Live users, who have downloaded 45 million things off of Xbox Live. Furthermore, the attach rate for Xbox Live Arcade titles is over 7 titles per user, on average. Satchell points out that the Xbox Live Arcade attachment number has doubled in the past four months.
Source: 1Up