Each year Fortune publishes a list of the 100 fastest-growing companies. The rankings are based on revenue, profits, and stock returns, and the list is limited to domestic and international businesses that trade on major U.S. stock exchanges. In the 2021 report, Etsy (ETSY -0.48%) and The Trade Desk (TTD -6.23%) placed fifth and sixth, respectively, but they ranked as the top two tech companies.

That recognition is certainly prestigious, but are both stocks still worth buying? Let's take a closer look.

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1. Etsy

Etsy has carved out a niche in the e-commerce industry. Its online marketplace specializes in unique and creative goods. Many items on the platform can even be personalized or customized for the buyer. That distinguishes it from other retailers, and it has helped Etsy become the sixth-most-popular online marketplace in the world in terms of traffic.

The secret to Etsy's success is its ecosystem of 5.3 million active sellers. That diversity keeps its marketplace filled with trendy products, from home furnishings and apparel to cosmetics and craft supplies. It has also brought 90 million active buyers to the platform. In fact, Etsy has doubled its base of active buyers and sellers in the past two years, underscoring the powerful network effect that drives its business.

Financially, Etsy has delivered spectacular results throughout the pandemic, and while growth did slow last year, the company is still growing quickly. Revenue rose 35% to $2.3 billion and earnings climbed 30% to $3.41 per diluted share.

Currently, management puts its core market opportunity at $466 billion, but that figure doesn't include the impact of two key acquisitions made last year. Etsy strengthened its presence in the apparel space with its purchase of fashion resale platform Depop, and it expanded into Latin America with its purchase of Elo7, a Brazilian marketplace that specializes in custom and made-to-order merchandise.

Over the past three years, shares of Etsy have climbed 77%, outpacing the 51% return of the S&P 500. And thanks to its differentiated business model and big market opportunity, I think Etsy will continue to outperform in the years ahead. From that perspective, this rapidly growing tech stock is still a buy.

2. The Trade Desk

The Trade Desk specializes in digital advertising. Its demand-side platform (DSP) leans on artificial intelligence to help ad buyers create, measure, and optimize data-driven campaigns. That content reaches viewers through channels like display, video, and social media, across devices like desktop, mobile, and connected TV.

Despite tough competition, The Trade Desk has distinguished itself in two key ways. First, it works only with ad buyers. That eliminates the conflict of interest faced by rivals like Alphabet, a company that simultaneously serves ad buyers and sellers, while also selling its own ad inventory from Google Search and YouTube. Second, its platform is built on bid-factor-based architecture, which allows advertisers to set more expressive targeting parameters than the line-item-based architecture used by every other buy-side platform.

Thanks to that competitive edge, The Trade Desk has become the largest independent DSP in the ad tech industry, and that scale has kept its customer retention rate over 95% for the last eight years. Not surprisingly, the company is growing quickly. Revenue rose 43% to $1.2 billion in 2021, an acceleration from 26% growth in the previous year, and non-GAAP earnings rose 32% to $0.91 per diluted share.

Going forward, The Trade Desk is well-positioned to maintain that momentum. Digital ad spend is expected to hit $571 billion in 2022, according to eMarketer, and management is working to capitalize on that opportunity. For instance, The Trade Desk recently introduced OpenPath, a tool that allows publishers to integrate directly with its DSP, rather than going through a supply-side platform like Google Ad Manager. Put another way, OpenPath will make ad transactions more efficient by eliminating unnecessary intermediaries.

Over the past three years, The Trade Desk stock has climbed 217%, crushing the performance of the S&P 500. Given the company's strong competitive edge and massive market opportunity, I think this stock will continue to outperform in the years ahead.