A world of ubiquitous computers was supposed to be the death knell of dowdy industries like direct-mail marketing, but the reverse seems to be true.

While Americans are sending a third less mail than they did 10 years ago, the junk-mail business is staying afloat, thanks to changes in databases, computing power and available storage.

Using Big Data—a catchall phrase for a combination of analytic software and huge computer storage—direct marketers have been able to refine their pitches with a newly startling precision.

A decade ago, these companies might have had 10 pieces of information about each individual household. The new approach allows direct marketers to immediately comb through hundreds of sources of public and private data and assemble more specific demographics, such as young, upper middle-class families with teenage kids who like gadgets.

"Instead of taking a month, you're talking about sub-seconds, in some cases" to run a query on the data, said Christian Ward, chief data officer at Infogroup Inc., which sells targeted consumer lists to retailers conducting direct-mail campaigns.