We're now halfway through 2019, and the markets are rolling. The S&P 500 is up a hearty 17%, but some stocks have fared even better through the first six months of the year. 

Roku (ROKU 0.68%)Snap (SNAP 5.62%), and Shopify (SHOP 2.40%) have all more than doubled in 2019. The year-to-date gains are impressive, so let's break down the reasons why investors are rallying behind these three stocks.

Shopify storefront on a smartphone and desktop.

Image source: Shopify.

Roku -- Up 196%

We're streaming video more than ever, and Roku is more than happy to be a crafty toll collector behind the scenes. There are now 29 million active users on its platform either through the online-tethered gadgetry that put Roku on the map or the growing number of smart TVs rolling out with Roku's proprietary operating system. 

Roku users streamed a whopping 8.9 billion hours in its latest quarter, helping Roku boost its average revenue per user by 27%. Roku users may treat the platform as a passive yet sticky pastime, but it's collecting money behind the scenes, both from the services that users subscribe to through its platform as well as from the growing number of ads it's able to crank out. Roku shares have nearly tripled this year. It's highly unlikely to nearly triple again through the second half of the year, but as long as growth continues at a torrid pace -- and the platform revenue that has overtaken hardware sales to be the primary driver here soared 79% in the first quarter -- it's hard to deny the bullish momentum. 

Snap -- Up 160%

Snapchat had a rough 2018, and the same can be said about its parent company Snap, Inc. A poorly received Android app update and a gradually sliding active user base scared investors away last year, but it's been a different story in 2019. 

Snapchat's decline in active users has bottomed out, and it's now connecting a growing audience of 190 million people. Snap is rebuilding its app, and it's making the platform itself more engaging through new filters, content, and mobile gaming diversions. Revenue rose 39% in its latest quarter, accelerating for the first time in more than a year. Social platforms rarely recover after they start to fall out of favor, but Snap is on the right path to be the exception to the rule.

Shopify -- 117%

More than 800,000 merchants rely on Shopify to get their storefronts online, The fast-growing hub for online retailers initially earned a living charging digital rent for its intuitive and reliable platform, but now it's cashing in on other revenue streams that are possible once you have earned the trust of your business-hungry customers. 

Shopify is often seen as a cyberspace oasis for budding entrepreneurs, but Shopify Plus -- its platform for larger merchants -- is growing even faster these days. Revenue rose 50% in its latest quarterly report, with adjusted earnings more than doubling. 

Roku and Snap tumbled sharply in 2018, bouncing back in 2019 off depressed levels. Shopify's strong first half of 2019 is impressive because the stock also beat the market last year with a 37% surge in 2018. Each company carved a different path to get to where it is today, but all three are clearly among the market's biggest winners so far in 2019.