Facebook’s $50 billion valuation promises a huge payday for investors — but first the social-networking phenom will have to prove it can earn the big bucks.

The lofty valuation comes courtesy of Goldman Sachs and Russia’s Digital Sky Technologies, which invested a combined $500 million in Facebook. That deal should have investors tripping over each other for a piece of the red-hot tech company when it sells shares to the public, expected some time next year.

But Facebook chief Mark Zuckerberg, who earned bragging rights as Time magazine’s “Person of the Year,” is under pressure to show that he can turn the site’s 500 million-plus users into digital dollars.

Facebook, with revenue between $1.3 billion and $2 billion, is now worth twice Yahoo’s $22 billion market cap, even though the aging portal’s sales of $6 billion are three times that of Facebook. Comparisons to China’s own social networking site, Tencent, which has revenue of $3.9 billion and a valuation of $38.5 billion, leave Zuckerberg with some headroom.

“These guys are masters at generating traffic,” said one media chief at an investment bank. “The question is how effective are they at monetizing it?”

Facebook’s main source of revenue is advertising, which has been the traditional road to riches for Internet companies. Mike Lazerow, CEO of Buddy Media, which gives advertisers tools for managing their brands on social media sites, said all of his clients are increasing their Facebook budgets.

But tech experts predict the social-networking site will find other ways — including expanding into new areas, from shopping to gaming — to crank up the cash machine.

Rather than entering credit-card details on scores of sites, Facebook Credits could become the currency of the Internet. Facebook, which gets a 30 percent transaction fee when its online currency is used, could expand it to e-commerce.

“They could provide a shopping mall and you can use their payment system,” said Lazerow. “They’ll make shopping more social.”

Facebook allows consumers to share their product “likes” with friends and could let people ship goods to friends without actually having their addresses.

Companies are already testing the e-commerce waters through Facebook. In June, for instance, Disney tested selling movie tickets through the site.

Facebook houses many users’ everyday photo collections. What if Facebook offered print services, or a cloud-based storage locker for $10 a month? Sources said the company is working on automatic photo identification or tagging of headshots posted to its site.

Facebook’s gaming business, which is built largely on Zynga games such as Cityville and Farmville, is also growing. The firm just snagged News Corp.’s Sean Ryan to lead its online gaming initiatives.

Many tech watchers believe Facebook is becoming more than a site, that it is an app that exists all over the Web.

“I think Facebook will be around just like the phone company,” said Buddy Media’s Lazerow. “They have the potential to be the biggest company in the world.”

catkinson@nypost.com