Yammer Founder David Sacks Joins Cloud HR Startup Zenefits As COO

David Sacks is back up in the startup game. The founder of Yammer, who sold his company to Microsoft for $1.2 billion about two-and-a-half years ago, has decided he’s had enough time off and is joining fast-growing cloud HR startup Zenefits as its new COO.

Sacks has been a Valley mainstay for the last 15 years. He joined PayPal as its first COO in 1999, leaving his management consultant job at McKinsey to become a member of the PayPal Mafia. In 2006 he founded genealogy website Geni.com, which was sold to MyHeritage in 2012, but not before the company’s internal communications tool Yammer was spun out. And we all know how that turned out.

So if you’re Sacks and you sold your last company for more than a billion dollars, why do you join Zenefits? Or really, go back to work at all?

“There’s only so much poker I can play,” Sacks said in a phone interview.

More seriously, Sacks says he’s spent the last few years angel investing and even flirted with the idea of becoming a full-time VC. But when push came to shove, he realized what he really wanted to do was go back to working on companies and building things.

The new role at Zenefits will pose a unique challenge for Sacks. Unlike his previous startups, which he either founded himself or got it at the ground floor, Zenefits has also scaled pretty substantially over the past few years. Being a part of that hyper-growth environment is part of what excites him about joining the company.

“What took Yammer three years to do, Zenefits did in one year… I’ve got experience in product and operations and issues related to hyper growth, and there’s a version of that here,” Sacks said.

When talking about the hire, Zenefits co-founder and CEO Parker Conrad said he sees Sacks as a partner in the business. And according to Conrad, all operations, finance, and product will report up through the new COO. That said, he also acknowledged that Sacks’ background at PayPal and Yammer will help to manage Zenefits through this hyper-growth phase.

“I expect David will be working on lots of different problems,” Conrad said. “Wherever the fires are hottest is where he’ll be focused.”

While he’s thrilled to have Sacks on board, Conrad said Zenefits wasn’t necessarily looking for a COO at the time discussions began. The conversation began after Conrad spoke with Andreessen Horowitz general partner and Zenefits board member Lars Dalgaard about the possibility.

As Conrad tells it, Dalgaard asked if he had considered adding a COO to reduce some of the pressure of running such a fast-growing company. But Conrad was pretty opposed to the idea until Sacks was mentioned as a possible candidate.

“I didn’t want a big company bureaucrat,” Conrad says. “But Lars asked, ‘What if it was David Sacks?’ and I was like, ‘You mean, Yammer David Sacks?'”

An intro was made soon after, with Conrad and Sacks first meeting the week before Thanksgiving. After spending several days together, it was clear they both wanted to be on the same team. Soon after they reached an agreement whereby Sacks would not only join as COO but also made an investment in the company and join the Zenefits board.

Sacks isn’t disclosing the size of the investment, but did say it was significant. “This is the biggest investment I’ve made in a startup, and it represents my commitment to the company,” he told me.

The investment shows Sacks has some skin in the game, but he says it’s secondary to the amount of work he expects to do as a part of the team. “I’ve been angel investing in companies for a long time, so that part isn’t unique… I just think this company is so exciting and special that I want to spend all my time on it,” Sacks said.