Just six months after the start of development, on May 5th, 2003, LinkedIn officially launched. After the first week, the site had registered close to 12.5K users.[1] Still, according to the company’s own numbers, growth was initially slow, and some days saw as few as twenty signups.[7]

Nevertheless, within four months, LinkedIn had hit the 50,000 user mark[1], and the company, behind the bonafides of its founding team and early promise, landed $4.7M in venture capital from Sequoia Capital in its first major financing round.[7][12] Within a year of launch, they’d reached 500,000 users.[1] In 2006, just three years post-launch, they achieved profitability. Then in 2011, eight years after their initial launch, LinkedIn became a publicly traded company

So how did LinkedIn grow from as few as 20 signups on some of those early days, to where they are today, almost eleven years later, as a publicly traded company with a market cap of nearly $18 billion, and a network of more than 225 million users from 200 countries and 4,800 employees

A 2004 Forbes article sums up the site’s core concept nicely, explaining that in the business world, “the most critical step towards finding a job, finding an employee or finding a business partner is getting a high quality referral.”[3] Yet referrals aren’t always easy to come by, especially because people typically have no way of knowing who their connections know. LinkedIn draws upon the theory of six degrees of separation—or, the idea that people are six introductions away from anyone they want to meet.[3]

As they’ve publicly acknowledged, the theory of six degrees of separation—the core concept informing LinkedIn—was not original to Hoffman and the team. Rather, it originated from Andrew Weinrich as the principle behind the networking site SixDegrees.com, which launched in 1997. Weinrich was granted a patent on the six-degrees concept in 2001. Six Degrees grew to 3.5 million users before being sold, at the height of the dotcom boom, to Youthstream Media Networks for $125 million in stock.

Within three months, however, the site had been shut down, and it wasn’t long after that until Youthstream was out of business entirely. Nevertheless, the concept behind the site had proven to be worthwhile, which is why LinkedIn, along with others, acquired the sixdegrees patent for $700,000 in 2003. It forms the basis of their IPO portfolio.

Although they took their core concept from Six Degrees, LinkedIn differentiated themselves from the defunct site as well as other social networking sites popular at the time. Rather than letting users initiate contact with anyone and everyone (as was the case for Friendster, the most popular network at the time), LinkedIn users would be restricted to trusted connections that they knew personally or were referred to through others in their professional network.

This gated-connection system helped curb spam and unsolicited invites, making LinkedIn a safe place for sought-after executives like Marc Andreessen, Jerry Yang, and Pierre Omidyar[3] to participate without being inundated with unsolicited requests, while still allowing the site to function as a powerful tool for users to get introductions through their professional networks.