IIRC, the active population hovers around 10k people. I do not believe IB has ever purged the user rolls.

So that means that 500k people includes spammers of all kinds, people joining for contests, and those who registered 12 years ago, asked a single question, and haven't been seen since

IHMO, the user rolls should be purged after a certain amount of years of no activity - certainly someone who joined in 1998 and hasn't posted since 1999 shouldn't be counted as a user any longer.