Now Monk-e-Mail.com has added a send-to-mobile option compatible with any handset and major carriers including AT&T, Verizon, and Sprint. Negotiations are still underway for distribution through T-Mobile.

"I don't think we ever thought Monk-e-Mail would explode the way it did online," says Cynthia McIntyre, senior director of advertising at CareerBuilder. "So it made sense to take it to the third screen, which is mobile."

The viral effort originally stemmed from the job site's "Working with Monkeys" TV campaign created by Chicago agency Cramer-Krasselt and introduced during the 2005 Super Bowl. Monk-e-Mail was started a year later to give the campaign a Web 2.0 twist. Since then, people have used the application created by ad technology firm OddCast to send 110 milion messages, with a pass-along rate of 20% to 25%.

CareerBuilder is hoping for similar success in the mobile realm, but has no set goals for usage.

"This whole world of viral initiatives is still sort of experimental marketing territory," says McIntyre. She adds that the sponsored Monk-e-Mails have helped the company build brand awareness and even bolstered sales efforts.

"We get stories from our sales force about people using Monk-e-Mail to get a foot in the door with clients they couldn't get appointments with before," says McIntyre.

Now that's monkey business.