Marketers are increasingly looking to smartphones and tablets to reach consumers, as spending on mobile advertising more than doubled during the first half of this year. According to new estimates from the Interactive Advertising Bureau, mobile-ad spending in the U.S. totaled $3 billion in the first half of 2013, up from $1.2 billion the previous year. In related news, research firm IHS projects that the number of Internet-connected video devices will pass 8 billion by 2017, a figure that should also be of interest to advertisers.

“Until recently, advertisers were reluctant to put much money into the mobile category, concerned about technological hurdles and the difficulties of measuring audiences,” reports The Wall Street Journal. “That hesitation is fading, ad executives say.”

Shifting ad dollars to mobile is happening quickly, in response to the amount of time consumers are spending with their devices. Unilever, for example, plans to triple its mobile ad outlays from last year (Unilever’s brands include Dove, Vaseline, Hellmann’s and Slim-Fast). Mondelez International, whose snack brands include Nabisco, Nilla Wafers and Trident, expects to devote 10 percent of its global marketing budget to mobile this year, including deals with Google, Twitter and Facebook.

“Since Apple introduced the iPhone in 2007 and the iPad in 2010, smartphones and tablets have increasingly supplanted laptops, desktop computers and television sets as the devices on which people surf the Web, communicate and watch video entertainment,” notes WSJ. “Adults in the U.S. are expected to spend an average of two hours and 21 minutes a day on smartphones and tablets this year, excluding time spent talking on phones, according to a recent study by eMarketer.”

Google anticipated the mobile-ad shift when it developed Android in late 2007 and offered it to smartphone manufacturers for free. Today, Android leads the mobile market, and eMarketer estimates Google will land 46.8 percent of the U.S. mobile ad market this year.

Facebook has been working on its mobile-ad products and is starting to see results. Mobile accounted for 41 percent of its ad sales in the second quarter this year.

Measuring audiences for mobile ads remains a challenge, as does establishing a standard for tracking, and launching approaches that do not alienate consumers. Additionally, cookies do not work the same on mobile devices as they do on the Web. While mobile is still in many ways an emerging platform with a learning curve, it seems marketers are catching on to its relevance.

“Marketers who wait to jump into the mobile pool until the last minute will find their competitors swimming like Michael Phelps while they’re wearing water wings in the shallow end,” suggests Dana Anderson, Mondelez SVP of marketing strategy and communications.

In a related story, new data from research firm IHS suggests the number of Internet-connected video devices will exceed 8 billion by 2017.

“The installed base of Internet-connected devices that can play video — which includes tablets, connected TVs, gaming consoles, smartphones, set-tops, Blu-ray Disc players and PCs — will hit 8.2 billion by 2017, a 90 percent increase from the 4.3 billion expected out there by the end of the year,” reports Home Media Magazine.

“On average every human being in the world will possess more than one Internet-connected video device by the year 2017, a major milestone for the electronics market,” explained Merrick Kingston, senior analyst of broadband technology at IHS. “We’re quickly approaching a world where the average broadband household contains 10 connected, video-enabled devices. This means that each TV set installed in a broadband-equipped home will be surrounded by three Internet-connected devices.”