A long overdue pruning was done to the member database.
A member account would be deleted if any one of the following conditions were met:
1. While lurking is allowed, I think it's reasonable to delete a user if their account wasn't "active" for at least two years AND hadn't any made ANY posts since they joined.
2. All users from the banned group
3. All users that hadn't completed the registration (via confirmation email). This a good thing as it releases the email addresses in case of an irrecoverable registration error - and will allow them to re-register with the same email.
That said, more than 6000 accounts were deleted just as an FYI