"Shark Tank" investors have picked hundreds of start-ups to invest in through the hit show's nine seasons, but not every winner goes on to great success. Indeed, what's known as the "Shark Tank" effect — exposure to the show's more than 5 million viewers — can be a key early step in scaling a business. But for two standouts from the 2015 Shark Tank season, reaching tens of millions in retail sales by 2017 was a success built on an ingredient more fundamental and deeper than the TV win: friendship.

A 'mini' idea hatched in Vietnam

Lovepop, a greeting card company that makes 3-D pop-up paper sculptures, began when two naval architecture students took a trip to Vietnam. Upon seeing the paper art there, Wombi Rose and John Wise decided to marry it with their talent for ship designing and started Lovepop in 2013. "It's so cool, and it's also exactly the same way you design a ship," said Rose. "You take a 3-D shape; you splice it into planes. The way that those planes intersect, you draw lines and then you cut out the pieces and assemble them by hand. And this is that same concept but miniature."

Courtesy of Lovepop

Before their appearance on Shark Tank in December 2015, Wise and Rose sold $300,000 in cards in their first year and a half. They won $300,000 from Kevin O'Leary for a 15 percent stake. After the show aired, their sales soared to $1 million in the following month and $18.7 million in sales in 2017, a sales bump that helped to earn Lovepop a spot on the new CNBC Upstart 100 list of promising young start-up companies. More from Upstart 100:

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How a tiny company that explains the news got Ryan Seacrest and Jimmy Iovine to invest "We decided very early that we were always going to put our friendship first before the business, and it's the only reason we've been able to weather the incredible ups and downs," Rose said. "Keeping our relationship healthy is what keeps the company healthy." To date, Lovepop has sold 8 million cards, and their goal is ambitious: to get to 1 billion. "I know every single one of those Lovepops will end up on someone's desk or table," Rose said. "[Customers] add so much personal meaning and emotion to these pieces of art. So it's not just a number, its an incredible set of experiences that are being created with our product. That's the most exciting, gratifying part of what we do."

An 'aha moment' over the watercooler

Glow Recipe, a beauty products company inspired by the South Korean beauty trend known as K-beauty, has also gone from a 2015 Shark Tank win to a spot on the 2018 CNBC Upstart 100 list. Similar to the friendship forged by the naval students who came up with Lovepop, co-founders Christine Chang and Sarah Lee began Glow Recipe after 20 years combined working at L'Oreal. Putting the pre-business relationship first was most important, and their friendship helped keep the start-up on course with their original mission. "We actually first met in L'Oreal Korea and coincidentally ended up working in the New York office," Lee said. "So it's been an interesting friendship also as co-workers. We were both looking at Korea, where all the innovative ideas were coming from, and when we were catching up one day, we had this "aha moment." We knew we had the bicultural and bilingual background and had access to the Korean manufacturers and markets while understanding the American consumers and retailers." The company won a $425,000 investment from Robert Herjavec on the show, but after a few conversations they decided not to accept the investment and parted ways amicably. But they still credit their "Shark Tank" appearance with helping their company. This year sales of its products at Sephora are expected to reach $30 million. In 2017, Glow Recipe's watermelon face mask was one of the best-selling products for the retail cosmetics chain, which is owned by luxury giant LVMH. Glow Recipe also has partnerships with Target and QVC.

Co-founders of K-beauty company Glow Recipe, Sarah Lee and Christine Chang. The former L'Oreal executives' watermelon face mask has become one of the best-selling products at Sephora. Lilly Lawrence | Getty Images Entertainment | Getty Images