Now, Greenspan (27), who like Facebook founder Zuckerberg moved to Palo Alto after college, is launching a new high-tech venture.
And while he recognises the odds of matching his former classmate's success are slim, he believes his service could be world-altering in its own way.
The service, called FaceCash, turns smart phones into digital debit cards that can be swiped at cash registers to make purchases.
Greenspan hopes to persuade retailers to ditch their debit card readers and point-of-sale systems for a laptop and a barcode scanner.
Unlike a swarm of other mobile payment systems, FaceCash doesn't rely on the existing credit card companies.
It seeks to replace them.
Greenspan's pitch to business owners is that he charges much lower transaction fees than, say, Visa or MasterCard.
For customers, the iPhone or Blackberry can replace a wallet full of credit cards and retail club cards, while offering features such as paperless receipts and real-time tracking of account balances.
The "face" in FaceCash refers to the security feature.
The phone's screen displays a photo of its owner above the barcode, eliminating the need for a signed receipt.
Greenspan has started small, by installing his software at a handful of Palo Alto eateries.
FaceCash applications will be available for iPhone, Blackberry and Android.
Greenspan may be starting local, but he's thinking big.
If it takes off, he thinks the FaceCash concept has the potential to render physical credit and debit cards obsolete.
It's an ambitious goal for a man who has already known disappointment and frustration in his brief entrepreneurial career.
If the similarities between Greenspan and Zuckerberg are obvious, the contrasts are nearly as conspicuous.
While the buoyant Zuckerberg has zoomed to billionaire-dom, Greenspan carries the weary pragmatism of someone labelled "bitter" in the national media before his 25th birthday.
When Zuckerberg takes the stage at big tech events, his uniform of jeans, sandals and a fleece jacket comes across as a sort of boyish anti-fashion statement.
Greenspan, with wonkish glasses and hair that's thinning at the back, wears a similar outfit like a man who knows no-one is looking at him.
Back at Harvard, when the two discussed their visions for the service that would become Facebook, Zuckerberg reportedly called Greenspan's version "too useful".
The distaste is mutual.
Sitting at a Red Mango frozen yoghurt shop after installing his system there, Greenspan shook his head at the direction Zuckerberg has taken the concept, calling it "extremely popular and extremely useless".
In keeping with his utilitarian focus, Greenspan's own first start-up focused on accounting software.
The mind-dulling hours he spent typing in receipts for his few clients gave Greenspan the idea for FaceCash, which digitises that process.
Susan Kuo, who bought the Red Mango franchise from Survivor: Cook Islands winner Yul Kwon earlier this year, said she was excited about the possibilities.
She would run FaceCash on a standard Dell computer, cheaper and less finicky than the $US1500 ($NZ2104) Micros touchscreen workstations she used for credit card transactions.
And then there are the lower transaction fees.
But the installation at Red Mango also revealed some kinks that still need to be worked out.
FaceCash requires a business's employees to sign up for its service on the web in order to log in and use it behind the counter.
Kuo said some of her workers don't have email, and those who do won't be eager to deposit the minimum $20 required to open a FaceCash account.
Over the weekend following the installation, Greenspan responded by creating an alternate sign-up process for employees.
His plan, he said, is to build a following among tech-savvy Palo Altans and Stanford students before expanding.
Helping him sell the concept is account manager Michael Anderson, a fresh-faced 19-year-old Palo Alto High School graduate who has taken time off from Yale to work on Greenspan's project.
Greenspan wasn't seeking venture capital funding because, he said, he didn't need it.
He noted drily he couldn't get venture capitalists to listen to him in the past, but several had come calling since the blog VentureBeat posted a favorable article about FaceCash on April 19.
Of course, it will take more than hype to present a real challenge to the big credit card companies.
The FaceCash concept sounded intriguing but would be difficult to pull off, said Sean Foote, a partner with Labrador Ventures and a venture capital professor at University of California-Berkeley's Haas School of Business.
The key, he added, would be getting a critical mass of retailers on board.
"For the consumer the challenge is, until he has signed up a significant percentage of my retail spending, then he's got to change my behavior from pulling out a credit card - which is fairly easy to do - to pulling out a cellphone," Foote said.
- Palo Alto Daily News
