Advertising revenue from direct mail is expected to plunge 39%, from $49.7 billion in annual spending in 2008 to $29.8 billion, by the end of 2013, according to a new forecast from Borrell Associates. If the forecast turns out to be right, direct mail would fall from the No. 1 position for ad revenue to No. 4, behind the Internet, broadcast TV and newspapers, the report added.