Online advertisers are expected to outspend TV advertisers by $40 billion this year. That means 40 percent of the world's ad spending is expected to take place online in 2018, according to new forecasts from advertising measurement company Zenith.

Online ad spending first beat out TV as the biggest ad medium last year.

Internet ad growth is being driven by social and video display ads, like those found on Facebook and YouTube. Globally, social media ad spending is estimated to rise 21 percent to $58 billion while video ad spending is rising 19 percent to $32 billion in 2018.

At 42 percent of total spending, search ads like those on Google remain the biggest form of online advertising, expected to reach $95 billion this year. Online search and classified spending is expected to grow less than 10 percent.

No surprise, all of this growth is taking place on mobile, which grew 25 percent this year to 60 percent of internet ad spending and 24 percent of total ad spending. Meanwhile, desktop ad spending declined nearly 4 percent.

As a whole, Zenith predicts global ad spending to rise about 5 percent, reaching $579 billion at the end of the year. That growth was revised upward from the 4 percent Zenith predicted in December and represents the biggest quarterly upgrade the company has made in seven years, thanks to upward momentum from markets in China, the Philippines, Argentina and Ireland.