So yesterday I met-up with one of the members of the PayPal Mafia, David Sacks.

FORBES: The LinkedIn IPO: A Valuable Lesson for Entrepreneurs

"Groups are the key, " says David Sacks, chief executive of San Francisco-based Yammer, which has four million users.

WSJ: The Business World Welcomes Social Networking

Zachary first met CEO David Sacks through his friend Elon Musk, who worked with Sacks at PayPal.

FORBES: Before $1.2 Billion Deal, Yammer Was Side Project: Largest Investor George Zachary

And when David Sacks is involved in a venture, its product is probably one that we should be using.

FORBES: Twist Mobile App: Good Things Come to Those Who Aren't Late

Yammer Chief Executive David Sacks said in a recent conference call that the company did the deal because it was a great fit with Microsoft.

WSJ: Pickup in Silicon Valley M&A Activity May Loom

"Salesforce will be a tab inside Yammer, " boasts cofounder David Sacks.

FORBES: No Story Type

David Sacks, who left PayPal in 2002 and now runs Yammer, a social-network site for businesses, met Thiel at Stanford, where they were members of the same eating club.

NEWYORKER: No Death, No Taxes

In scenes resembling a relative Christmas Day, lonely David will be opening packs of toilet tissues, large sacks of potatoes, cabbages, carrots and large boxes of Corn Flaks during the afternoon.

BBC: Castaway valentine couple reunited