About 1.3 million malicious ads are being viewed online everyday, most pushing drive-by downloads and fake security software. 97% of Fortune 500 web sites are at a high risk of getting infected with malware due to external partners (such as javascript widget providers, ad networks, and/or packaged software providers) The probability of a user getting infected from a malvertisement is twice as likely on a weekend and the average lifetime of a malvertisements is 7.3 days. The New York Times and Gawker media are among the victims.
Source: https://threatpost.com/research-13-million-malicious-ads-viewed-daily-051910/73989/