The letter was signed "Mom and Dad" but it was aimed, of course, at the world. Mark Zuckerberg's letter announcing that he and Priscilla Chan were giving away 99 per cent of their Facebook shares is just over 2200 words long.
That's room for a lot of detail - and he seems to offer it. He specifies the amount of the gift: US$45 billion ($68 billion) at current market value. He emphasises the need for long-term solutions. He suggests that advancing human potential and promoting equality "will require a new approach for all".
The US$45 billion exceeds the Gates Foundation's current endowment. It's larger than Warren Buffett's US$30 billion gift to the Gates Foundation in 2006, then the largest charitable gift in history.
The world, understandably, went apeshit. Or, to put a sociological gloss on things, we experienced "collective effervescence", sociologist Emile Durkheim's term for rare moments of community cohesion marked by mass outpourings of a shared sentiment.
There were some early sceptics, those quick to impute a selfish motive. But they were soon deafened by the global chorus of well-wishers chanting the same refrain: thank you, Mr Zuckerberg. Thank you for your revolutionary gift. It turns out neither assumption was right. There was no gift. Nor is there anything particularly revolutionary about Mr Zuckerberg's new business model.
Whether you call it charity or philanthropy, one thing is clear: Mr Zuckerberg didn't surrender anything. He set up a limited liability company called the Chan Zuckerberg Initiative, and transferred US$1 billion in Facebook shares to it. As Jesse Eisinger reported in the New York Times, he simply "moved money from one pocket to the other".
As if sensing that our newfound effervescence had fizzled rather abruptly, a crack team of management scholars and business journalists tried to cheer us with a single message. You're right, they conceded. It's not charity. It's better than charity. It's a new, radical movement we like to call "philanthrocapitalism" - and it's going to make you all rich. How, you might ask? By giving more philanthropy to the wealthy.
The most eloquent scribe of the philanthrocapitalism movement is Felix Salmon, a once-sceptic turned believer who offered this homage to Mr Zuckerberg's new investment vehicle: "Mark Zuckerberg isn't going to be satisfied with small, visible interventions which don't scale - feeding the hungry, say, or giving to the poor. Such activities improve the world, but they don't change the world."
Salmon's right to suggest that Mr Zuckerberg and other tech entrepreneurs often share an aversion to helping the poor directly. But he's mistaken in suggesting that philanthrocapitalism - which includes a dizzying array of new techniques for making money out of the poor - is novel. On the contrary, this phenomenon is simply an extension of a centuries-long effort to enrol every segment of the known universe into a realm where individuals can profiteer from relationships with other people.
The only thing new about the "new" philanthropy is that an older and stronger beast that's been chasing for years after the riches it can smell in the non-profit world has finally caught its prey and begun feasting. Philanthrocapitalism is what happens to charity after capitalists swallow it.
Three trends exemplify the philanthrocapitalist turn: the rise of microfinance; the growth of impact investing; and the rise of new investment vehicles such as Mr Zuckerberg's new limited liability, which structure themselves as for-profits so they can offer grants and investments to for-profit recipients without pesky disclosure and transparency requirements.
In each of these cases there is little direct evidence of positive outcomes for the global poor and considerable evidence that such trends tend to enrich the wealthy at the poor's expense.
Mr Zuckerberg is being hailed for his "unusual" decision to establish a business to channel his wealth into good deeds. But we shouldn't be dazzled by the appearance of novelty. Trying to blend good deeds with good deals is neither revolutionary nor unprecedented. It's simply another step on the dogged march to source new supplies of capital. It's business as usual, rebranding as philanthropy, and announced with a deceptive air of selflessness that left us near-grovelling in thanks.
Bill and Melinda Gates
Net worth: US$79.7b; US donations US$31.5b
The co-founder of Microsoft and the richest man in the world set up the Bill and Melinda Gates Foundation in 2000 with the goal of reducing inequality.
Net worth: US$40.5b; US donations US$3.7b
The business magnate and former New York City mayor is founder of the Bloomberg Foundation, which gives primarily to educational, health and environmental causes.
Net worth: US$66.7b; US donations US$30b
Buffett, the celebrated investor known for living a relatively frugal life, has pledged to give away 99 per cent of his wealth.
Sir Tom Hunter
Net worth: £1.2b; UK donations £1b
The Scottish retail tycoon, who began selling trainers from the back of a van, established the Hunter Foundation, an initiative to help educational and entrepreneurial projects in Scotland, and sustainable development in Africa.
Linsey McGoey is the author of No Such Thing as a Free Gift: The Gates Foundation and the Price of Philanthropy.