Top 10 MMORPGs by player numbers

1. World of Warcraft, released 2004 - 8.5 million subscribers.
2. Habbo Hotel, released 2000 - 7.5 million active users.
3. RuneScape, released 2001 - 5 million active users.
4. Club Penguin, released 2006 - 4 million active users.
5. Webkinz, released 2005 - 3.8 million active users.
6. Gaia Online, released 2003 - 2 million active users.
7. Guild Wars, released 2005 - 2 million active users.
8. Puzzle Pirates, released 2003 - 1.5 million active users.
9. Lineage I/II, released 1998 - 1 million subscribers.
10. Second Life, released 2003 - 500,000 active users.

Fonte: http://forevergeek.com