Excerpts:

As Karim later recalled, what he learned at PayPal could be summed up in two words: “Stay flexible.” That mantra would help guide YouTube.

PayPal’s culture encouraged entrepreneurship. "At PayPal, we first and foremost, hired people always looking for those that could [form companies], and many did,” Levchin told Mashable. “A key interview question was ‘are you thinking of starting your own company after this?’ Most said ‘yes.’ “

For Karim, two disparate incidents crystallized this problem: the Dec. 26, 2004 tsunami in Indonesia and Janet Jackson’s "wardrobe malfunction” during Super Bowl XXXVIII in January 2004. In the latter, Karim found that although everyone was talking about the incident, you couldn’t find a clip of it anywhere online. With the tsunami, Karim said he believed there were probably lots of videos of the disaster, but no way to access them.

But since there weren’t many videos on the site, Karim populated it with videos of 747s taking off and landing.

Desperate to get people on the site, YouTube ran ads on Craigslist in Los Angeles and Las Vegas, offering women $20 for every video they uploaded. Not a single woman replied.

Supan remembers a report from the time stating there were around 280 other video-sharing sites, many of which had been around before YouTube. She told Mashable, “We were basically last to market. Really no service launched as far as I could remember after YouTube.”

Botha said that much of YouTube’s success came because the site was so easy to use. Chen’s team had made sure you could load a video in any format to the site. “You could load it and they took care of transposing it back in Flash,” Botha said. YouTube also made it easy to cut and past a URL by making that feature prominent on the site. YouTube also had a public view count. As Twitter would later discover, public metrics can help grow your brand by appealing to users’ vanity.

Finally, YouTube also made it easy to embed videos, which many users opted to do on MySpace, the hot social network at the time.

“A lot of people asked ‘You make a website and people add their content and consume it and you make all the money?’ In 2005, 2006 these were the questions. Now it’s a given that people will contribute content. At the time, it wasn’t obvious.”

Supan has a slightly different view. She said the key to YouTube’s success was that the video player always worked. Period. “People don’t mind if products aren’t perfect, as long as they work,” she said. “You have to do one thing well.”

As Hurley recalled in a later interview, the inspiration for YouTube’s sharing functionality was once again PayPal, in particular a payment button bloggers and publishers could use on their own sites. “That button took them back to the PayPal experience,” Hurley said “We tried to do the same thing with a video solution.”

Over time, Google would evolve Video, focusing on improving its quality. In Google’s view, YouTube’s clips were low-quality and junky, sort of like the chaff it tried to cull from its searches. Supan said that missed the point. “Their biggest argument against YouTube was it was low-quality and grainy, and who wants to see that? Well, the world did.”

Botha recalls that after Google bought YouTube, he overheard a Google engineer complaining, “I don’t know how they won. Our video quality was so much higher.”

Billionaire Mark Cuban was outspoken at the time, stating that only a “moron” would buy YouTube.

Most people asked whether YouTube would recoup its investment. Many years later, $1.6 billion doesn’t seem like that much.“

YouTube made $1.13 billion in U.S. ad revenues from its videos in 2014, according to an eMarketer estimate. Globally, the figure is about six times that.