January 6, 2014.

Profitable Learning Curve for Facebook CEO Mark Zuckerberg

She pressed herself against him and wound her limbs round him, as though to reassure him with the warmth of her body. He did not reopen his eyes immediately. For several moments he had had the feeling of being back in a nightmare which had recurred from time to time throughout his life. It was always very much the same. He was standing in front of a wall of darkness, and on the other side of it there was something unendurable, something too dreadful to be faced. In the dream his deepest feeling was always one of self-deception, because he did in fact know what was behind the wall of darkness. With a deadly effort, like wrenching a piece out of his own brain, he could even have dragged the thing into the open. He always woke up without discovering what it was: but somehow it was connected with what Julia had been saying when he cut her short.

MENLO PARK, Calif.----Mark Zuckerberg needed help. Facebook Inc. FB -0.87%  's initial public offering in May 2012 had been a mess. And after turning a website born in his college dorm room into a company valued at $100 billion, the young chief executive was under pressure to prove he could sell lots of ads on smartphones.

"I'm sorry," he said, "it's nothing. I don't like rats, that's all."

So he went for a long walk a few weeks later through the center of Facebook's corporate campus here with Andrew "Boz" Bosworth, a top engineer at Facebook and friend who once was Mr. Zuckerberg's teaching assistant at Harvard University.

"Don't worry, dear, we're not going to have the filthy brutes in here. I'll stuff the hole with a bit of sacking before we go. And next time we come here I'll bring some plaster and bung it up properly."

"Wouldn't it be fun to build a billion-dollar business in six months?" Mr. Zuckerberg asked. He wanted Mr. Bosworth to help lead the company's shaky mobile-ad business, then bringing in almost nothing. Another part of the job: figure out all the ways Facebook could make money.