A distinct shift in e-reader demographics have occurred, which has implications for marketers considering this ad channel. Namely, the majority, or 61% of e-reader owners are now women, compared to 46% less than a year ago, Nielsen says. Tablet owners are 57% owned by men, only a slight decrease from the previous year. Smartphone owners, meanwhile, are split equally between men and women.

There are a number of reasons for the preference by women for e-readers, writes Biz Report. They are lighter, easier to carry and because they have one purpose only they provide a good distraction from work and family cares. There is also a measure of privacy involved that might have something to do with the preference, Biz Report speculates: you can read a book in public  with an e-reader and no one has to know its title or subject matter.

The Subway Effect

This has been noted before, albeit from a different perspective. Book publishers and authors have long relied on the free marketing a well-designed book cover offers, especially as they are carried around in public places. As digital books become more popular this group has been noticing the loss, the New York Times reported.

There is hope that Facebook could make up for this. or now, many publishers are counting on the Facebook effect. "Before, you might see three people reading ‘Eat, Pray, Love’ on the subway," Clare Ferraro, president of Viking and Plume, imprints of Penguin Group USA, told the Times. "Now you're going to log onto Facebook and see that three of your friends are reading ‘Eat, Pray, Love.’ "

A Growing Ad Format

For online marketers, though, this shift is good news - more e-book readers means more exposure to this ad category. EBooks are a high growth category, as sales are expected almost triple to $2.8 billion by 2015, according to Forrester Research. Amazon has been reporting that it is selling more copies of its Kindle ebooks than paperbacks.

E-reader ads received a big push earlier this year when Amazon.com unveiled a Kindle with software to show ads to readers. General Motors's Buick, Procter & Gamble’s Olay skin products and Visa were be among the first advertisers.