At first blush, you might think that Shopify (NYSE: SHOP) had an underwhelming third quarter. After all, the company -- which provides a platform for just about anyone to establish an e-commerce presence -- has seen its stock fall over 10% since announcing results. Nothing could be further from the truth. Growth stock investors should be reassured knowing that on virtually every metric, Shopify is winning over tons of customers. But there's one underappreciated metric that really points to how well the company is actually doing ... and will continue to do. Image source: Getty Images. First, the headline numbers We'll get to that metric in a moment. But let's recap Shopify's third quarter with the headline numbers. Metric Q3 2020 Q3 2019 Growth Subscription solutions revenue $245 million $166 million 48% Merchant solutions revenue $522 million $225 million 132% Total revenue $767 million $391 million 96% Gross merchandise volume (GMV) $30.9 billion $14.8 billion 109% Data source: SEC filings. Revenue figures rounded to the nearest million. Let's walk through this step by step. The value of all the "stuff" sold on sites built using Shopify's platform (GMV) more than doubled to $30.9 billion. Because of that, total revenue also almost doubled to $767 million. While subscriptions (what merchants pay to use the platform) showed solid 48% growth, merchant solutions (primarily payment options when a customer buys something) zoomed 132% higher. Put all these pieces together, and you have a company that is quickly becoming the most powerful North American e-commerce juggernaut not named Amazon. The most important metric When the pandemic hit, Shopify made it easier for merchants to sign up and kick the tires on the platform. It extended the window for free trials to 90 days, up from the standard 14-day trial. This provided enough time for businesses of all sizes to make a transition without incurring any costs. If those customers liked what they saw (and were able to stay in business), they would become paying customers. Or at least that's the narrative many investors (myself included) created. When the company revealed this tidbit about monthly recurring revenue (MRR), it became clear that narrative was true. Image source: Shopify investor relations. As you can see, MRR growth was slowing coming into the third quarter, but once that 90-day window for trials closed, it exploded to over $74 million. For those keeping track at home, that annualizes to almost $900 million in practically guaranteed revenue without a single sale taking place on the platform. What's more, the management pointed out on the earnings call that there would have been a record number of sign-ups even without the effect of the 90-day window closing. Shopify president Harley Finkelstein said, "In Q3, a record number of merchants became paid subscribers to Shopify, even excluding merchants who converted following the end of their 90-day extended free trial. The extended free trial made it easier for new stores to get online fast and make sales, enabling many of these entrepreneurs to generate cash while their physical stores were shut down at the onset of the pandemic." The moat gets wider Why does all this matter? Because Shopify has an enormous moat. Consider: High switching costs: When a smaller business sets up a website and social media presence via Shopify's platform, it doesn't want to waste any time switching to a new provider. Doing so is expensive and time consuming. It's also a distraction. Network effects: Shopify has a marketplace where third-party app developers (read: non-Shopify employees) can sell their tools. When they do, they get instant access to Shopify's stable of over one million merchants. The more merchants that join, the more incentivized those developers are to build on Shopify's platform -- attracting more merchants. There are currently over 5,300 such apps, many of which can't be found anywhere but Shopify. The bottom line: Once merchants enter Shopify's ecosystem, they're likely there for good. It's true many of these businesses will fail, but that's how a functioning economy is supposed to work. With so many merchants joining in such a short time, Shopify just wove an enormous web that promises to deliver even more successful merchants in the years to come. Investor takeaway Yes, Shopify stock has trended down in the past month. But no, you shouldn't worry too much. Let's not forget that the stock has nearly tripled since bottoming out in mid-March. We shouldn't be worrying too much over short-term moves. The company's MRR growth is a vital indicator that the pipeline for future growth is strong. With Shopify already accounting for over 14% of my real-life holdings, I won't be adding shares, but I surely won't be selling anytime soon, either. 10 stocks we like better than ShopifyWhen investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* David and Tom just revealed what they believe are the ten best stocks for investors to buy right now... and Shopify wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of October 20, 2020 John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool’s board of directors. Brian Stoffel owns shares of Amazon and Shopify. The Motley Fool owns shares of and recommends Amazon and Shopify and recommends the following options: short January 2022 $1940 calls on Amazon and long January 2022 $1920 calls on Amazon. The Motley Fool has a disclosure policy.Source