'Mobile shift is happening,' Facebook execs say

Facebook reported better-than-expected results across the board Wednesday, with 76 percent year-over-year ad growth in the fourth quarter.

Ahead of the company's earnings call I caught up with COO Sheryl Sandberg and CFO David Ebersman about what's driving the growth.

"This is obviously a really exciting quarter for Facebook ... the mobile shift is happening and products are working for marketers," Sandberg said in a phone call.

Facebook Chief Operating Officer Sheryl Sandberg with CNBC's Julia Boorstin in an interview in 2012.
Source: Facebook

Sandberg pointed to the seismic mobile shift in 2013—the first year people spent more time on mobile than on TV.

(Read more: Facebook beats; stock jumps)

Facebook "made a huge investment in newsfeed ads," she said, referring to those tailored to stream in mobile apps. "Marketers want to be where people are—that's driving mobile results."

Ebersman said, "The most important number for us is the 76 percent growth in ad revenue versus last year—a very strong data point that Facebook advertising works. We're seeing returns from all the investments we're making in great mobile products to attract users."

The ability to measure results is also driving growth, the executives said.

"If you look back four to five quarters ago, we couldn't prove to marketers that we were ringing the cash register," Sandberg said. "We couldn't say, 'Here's the difference in sales.' " Now, she added, the investment in measurement is paying off, and Facebook can prove results both in-store and online.

(Read more: 4 social media strategies to boost your business)

She said a French campaign for Coca-Cola's polar bears had a return on investment 3.6 higher than TV, and Facebook drove 27 percent of Coke's incremental sales.

Facebook ad revenues 'on fire': Pro
Facebook ad revenues 'on fire': Pro

Sandberg also stressed Facebook's growth in small and midsize business' ads—what she called the "holy grail," because they are so numerous and so hard to move online.

She and Ebersman both dodged questions about concerns that Facebook usage among young teens in the U.S. in waning.

"There's nothing new to say," Sandberg said. "Marketers who want to reach all audiences, including teens, are coming to Facebook."

Ebersman added, 'We're really pleased with continued strong growth in the size of the network of people who use Facebook. … We added well over 100 million daily and monthly active users last year." Those numbers don't include Instragram, he said, which is in the earliest phase of advertising.

—By CNBC's Julia Boorstin. Follow her on Twitter: @JBoorstin.