How many Gmail users are there exactly?

Just how popular is the invite-only Gmail service compared to all mail services and how many users does it really have? According to ComScore Networks (via Alex), Gmail currently has 7.1 million U.S. users, putting it in fourth place. Yahoo! Mail leads with 52 million U.S. users followed by AOL and Hotmail which each have about 30 million U.S. users. Worldwide, Yahoo! Mail and Hotmail lead the pack with 226 million and 222 million users respectively while Gmail trails in fourth place with 52 million worldwide users.