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In 2018, we saw 190 companies become listed on U.S. exchanges, and 2019 is proving to be no different. This year has been a hectic one for IPOs, with companies such as Uber (NYSE:UBER), Pinterest (NYSE:PINS), Beyond Meat (NASDAQ:BYND), and Slack (NYSE:WORK) going public so far. The year is far from over though, with many companies potentially going public in the next four months.

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These three companies plan on going public soon and should definitely be watched by investors.

1. Airbnb

Airbnb has taken the hotel and hospitality industry by storm. Launched over 10 years ago, it now hosts over 2 million people a night, for a total of 400 million people since the company was founded in 2008. Airbnb is available in 191 countries, 100,000 cities, and offers roughly 6 million different places to stay. Thanks to its cheap and easy-to-use service, Airbnb has become a household name.

In 2018, Airbnb saw a 62.5% increase in guest arrivals, with Mexico City and Beijing up 79% and 91%, respectively. Even though Airbnb is a privately traded company, reports have shown that in 2018's third quarter, Airbnb turned over $1 billion in revenue, with some analysts expecting revenue to hit $8.5 billion by 2021. A portion of this growth comes from Airbnb's more recent venture, Airbnb Experiences. Launched over three years ago, this service offers over 5,000 experiences in 60 cities across the globe, growing 2,500% in 2018. Unlike many companies that have gone public recently, Airbnb has also reported a positive earnings before interest, taxes, depreciation, and amortization (EBITDA) for the past two years. This has been aided by the experiences segment, with some experiences earning more than $200,000 each.

The growth of Airbnb has not gone unnoticed by traditional competitors such as (NASDAQ:BKNG) and TripAdvisor (NASDAQ:TRIP), which now face a significant threat. A recent survey from Morgan Stanley indicated that 42% of Airbnb users had replaced a traditional hotel service with Airbnb's digital service. This is no surprise, with hotels costing 6% to 17% more than an Airbnb.

While Airbnb has not yet announced when it intends to go public, an IPO is certainly on the table for the end of 2019. In a statement, co-founder Nathan Blecharczyk said: "We have already said that we are taking the steps to be ready to go public in 2019. That doesn't mean we will go public in 2019."

2. Instacart

Apoorva Mehta, CEO of the popular grocery delivery company Instacart, said at the start of this year that "an IPO is definitely on the horizon." Founded by a former Amazon employee, Instacart reached an $8 billion valuation in 2018 after raising $271 million in funding last November.

While online grocery shopping makes up only 10% of consumer spending, the industry has seen significant growth in recent times. From 2016 to 2018, the online grocery market doubled, and it is expected that between 2018 and 2023, the market will quadruple, with 1 in 5 Americans predicted to buy groceries online.

Although Instacart's revenue and profitability are not publicly disclosed, the company appears to be growing. Founded in 2012, Instacart now serves over 15,000 grocery stores in 4,000 cities and has formed partnerships with companies such as Kroger, ALDI, Sam's Club, and Walmart Canada. As of 2018, Instacart had over 30,000 customers and has been investing heavily in research and development, announcing the opening of a second R&D headquarters in Toronto, Canada.

In an industry as explosive as online grocery shopping, Instacart faces great competition from companies such as Walmart, Amazon, and Target. These popular companies have already carved out a name in the space, and it is apparent that customers will use their delivery service first. According to a study by Second Measure, Walmart -- one of Instacart's largest competitors -- has 62% more online shoppers. Amazon, through Prime Now and Amazon Fresh, currently makes up 30% of all online grocery shopping, and Target's Shipt saw 92% more orders in January 2019 compared to January 2018. Despite such intense competition, Instacart may have a significant role to play, providing smaller, regional retailers with logistical expertise and e-commerce options.

3. SmileDirectClub

SmileDirectClub is another company that will definitely go public in 2019 after filing a form S-1 in August. SmileDirectClub provides clear teeth aligners, a replacement for traditional train-track-style braces. Unlike their main competitor, Align Technology's (NASDAQ:ALGN) Invisalign, however, SmileDirectClub's aligners can be fitted either at home or in "SmileShop" stores, and thus cost roughly 60% less than Invisalign.

This affordability and ease of use has made them a significant threat to Align, which actually owns 19% of SmileDirectClub. Despite being part owners, their relationship has not always been healthy, with Align forced to close its physical stores due to a non-compete violation with SmileDirectClub. A deal struck last year between SmileDirectClub and CVS will see the products offered in hundreds of CVS stores across the U.S. This partnership will increase customer awareness for SmileDirectClub and should prove to be a big success against competitors.

SmileDirectClub recently received a valuation of $3.2 billion and has had over 300,000 customers since its founding in 2014. With the global orthodontics market set to reach $2.6 billion in 2023, it is no surprise that SmileDirectClub has a lot of room to grow, especially at a time when consumers demand things cheaply and efficiently. In 2018, SmileDirectClub's revenue grew 184% to $398 million.

Although SmileDirectClub has a unique position in the market and is seeing huge revenue growth, investors should be wary as dentists have taken issue with its product. Dentists have filed complaints in 36 states as they believe the process of teeth straightening is an important medical procedure and requires numerous checkups. Government intervention in the dental industry could prove to be dangerous for SmileDirectClub's business model. Nevertheless, this soon-to-be-public company is seeing impressive growth, and investors should definitely keep a close eye.

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.