WHERE ARE THEY NOW: Facebook's young billionaires

Net worth: estimated at $850 million

Age: 31

After leaving Facebook in 2007, Hughes worked on digital strategy for Barack Obama's 2008 campaign and founded Jumo, a nonprofit social network that has since merged with the GOOD organization.

In 2012, Hughes purchased a majority stake in the New Republic, a move that has been controversial in the media industry.

He is married to former New York congressional candidate Sean Eldridge. Together they own an 80-acre estate in Garrison, New York, a two-bedroom home in Shokan, New York, and a loft in New York City's Soho neighborhood. The apartment is currently on the market for $8.75 million.

Net worth: $9.1 billion

Age: 31

Moskovitz left Facebook in 2008 to start task-management app Asana, which he runs full-time to this day.

He and his wife Cari have donated millions of dollars to causes like malaria eradication and marriage rights through Good Ventures. They are also signers of the Giving Pledge, which means that they plan to give away at least half of their wealth.

In a 2013 Quora thread, Moskovitz described the responsibility he and his wife feel with regards to their wealth: "Cari and I are stewards of this capital. It's pooled up around us right now, but it belongs to the world. We are not perfect in applying this attitude, but we try very hard."

Net worth: $2.8 billion

Age: 35

Parker, who also founded Napster, recently announced a gift of $600 million to start the Parker Foundation, which will focus on funding programs in three main categories: life sciences, global public health, and civic engagement.

Parker recently pledged $24 million to develop the Sean N. Parker Center for Allergy Research at Stanford.

In 2014, he launched a civic engagement startup called Brigade Media.

In the summer of 2013, he spent some $4.5 million to transform a campsite in Big Sur, California, into a magical forest paradise for his wedding to Alexandra Lenas. Parker faced heavy criticism after the wedding, because the preparation required installing temporary structures in an ecologically sensitive area, and he failed to obtain the proper permits from the California Coastal Commission.

The controversy ended with a $2.5 million settlement with the commission.

Net worth: $1.12 billion

Age: 45

"Lean In," Sandberg's book about women in the workplace, became a bestseller. She's now telling men and women to "Lean In Together" and recently launched a campaign for gender equality with the NBA and WNBA.

Sandberg's husband, SurveyMonkey CEO David Goldberg, died unexpectedly in May of 2015. He was 47.

In July, the New York Times reported that Sandberg would be joining SurveyMonkey's board.

Net worth: $3.3 billion

Age: 53

Milner's Digital Sky Technologies invested $200 million in Facebook in 2009. The firm also took a huge stake in Alibaba in 2011, three years before the Chinese company's record-breaking IPO, and has invested in Xiaomi, Twitter, Airbnb, and Spotify.

Digital Sky Technologies sold all of its Facebook and Twitter shares in 2013 and 2014.

In 2011, Milner bought a 30,000-square-foot mansion in Los Altos Hills for a whopping $100 million.

Net worth: $2.2 billion

Age: 47

Thiel was Facebook's first major investor, joining the social network when it was still a tiny operation in 2006. Thiel sold most of his stake in Facebook in August 2012, shortly after the company's IPO. His pay-out was said to be worth just more than $1 billion.

Through his venture capital firm, Founders Fund, Thiel has made investments in startups like Airbnb, SpaceX, Spotify, and Stripe.

In 2011, Thiel set up a fellowship program that gives $100,000 to entrepreneurial college students to drop out of school and start their own companies.

Net worth: $38.6 billion

Age: 31

Most recently, Zuckerberg has led acquisitions of mobile messaging app WhatsApp, which has 700 million global users, and Oculus VR, whose virtual reality headset is currently under development with an expected launch in Q1 2016.

Zuckerberg and wife Priscilla Chan have been busy — in October 2014, they paid more than $100 million for a 700-acre property on the north shore of Kauai while simultaneously working on a renovation of their San Francisco home and fighting a law suit relating to their Palo Alto home.

In February 2015, they donated $75 million to a new trauma center for San Francisco General Hospital, which will be renamed in their honor.