500k is pretty good if it's stable (FFXI sat at that number for a while).
Not sure FF11 ever hit 500,000 players logging on daily. They hit 500,000 subs.

You can have an account, with a sub, and log in once or twice a week, too. Those wouldn't be counted. You know those people who log on Monday (now Tuesday) and don't log til the next one? Not counted.