Facebook will start selling stock publicly for the first time on Friday – making the company worth around $100bn. This will also make those lucky enough to have secured stock options very, very rich.

Founder and CEO Mark Zuckerberg is expected to see his personal wealth rise to around $15.5bn. But what will he do with all that money? More to the point: what should he do with it?

Where will all the money go?

Facebook's initial public offering (IPO) will make Zuckerberg, who just celebrated his 28th birthday, the richest person under 30 in the world. It'll also mean he can give away massive amounts of money. And he has his customer base, which now approaches some 15% of the world's population, to thank for it. How might he repay them?

Early signs were good: Zuckerberg made headlines in 2010 when he added his name to the band of billionaires, organised by Microsoft co-founder Bill Gates, who have pledged to give the majority of their wealth to charity.

Since then, however, we’ve heard little about his philanthropic plans – though there was a $100m donation to public schools in Newark, New Jersey.

And the pledge itself tells us little about the kinds of cause Zuckerberg might champion. The billionaires who have signed up don't pool their money or direct it to a specific cause – it's a "moral commitment" that has no legal weight.

A social mission?

Critics of the pledge have highlighted its uncertain terms, raising questions about who the money will benefit and how. As Aaron Dorfman, executive director of the National Committee for Responsive Philanthropy in Washington, put it: “In general, wealthy donors do not give in ways that benefit underserved communities." He highlighted a study that found that for every $3 given to charity, only $1 went to the underserved.

But a letter from Zuckerberg to prospective Facebook investors gives cause for hope. In it, he hints at a broader vision, writing that Facebook “was built to accomplish a social mission — to make the world more open and connected”.

In emphasising that “[w]e don’t build services to make money; we make money to build better services,” he has made some investors edgy – but also showed that his interest lies deeper than simply maximising profits.

Let's call on him to break the mould

Columnist Robert Frank warns that we are unlikely to see an immediate impact on charitable giving as a result of the IPO. He points out that “people typically don’t start thinking about this stuff (philanthropy) until they’re in [their] 50s or 60s".

But if Facebook's customers – the source of its riches – made it clear that charity should be a priority, would Zuckerberg buck the trend?

As Zuckerberg put it when he signed up to the Gates pledge: "People wait until late in their career to give back. But why wait when there is so much to be done? With a generation of younger folks who have thrived on the success of their companies, there is a big opportunity for many of us to give back earlier in our lifetime and see the impact of our philanthropic efforts."

While investors will be watching Zuckerberg's every move to track the value of their stake, the rest of us will be watching for something quite different.

Take action: Find out about the Facebook founder's road to riches, and post your own ideas about what he should do with $15.5bn in the comments below.

And if that's not enough, why not start your own Facebook petition to Mark Zuckerberg?