I think that the right title should be "Marketing Firm Exactis Exposed a Personal Info Database with with 340M Records on Internet". This is not a leak, at least there is no evidence of it yet. While this does not downplay this security "mishap", there is still big difference between "someone rob a bank" and "bank left their vaults open".
OTOH, it would be interesting to know how did they get hold on such data.