We may be at the midpoint of 2018, but some stocks have already had a banner year. There are several stocks that have more than doubled year to date, far outpacing the S&P 500's pedestrian 2% return in 2018. 

Netflix (NASDAQ:NFLX), Carvana (NYSE:CVNA), and Twilio (NYSE:TWLO) are just some of the names that have delivered game-changing wealth to investors through the first six months of 2018. Let's go over why these three stocks have more than doubled this year.

The Carvana vending machine tower in Tempe, Arizona.

Image source: Carvana.

Netflix: Up 103%

The top dog in premium streaming video has been on fire this year. Netflix continues to dazzle investors with spectacular growth, and the top-line gains are actually accelerating. Revenue growth topped 40% in the first quarter, Netflix's biggest percentage gain since 2011. Guidance calls for even headier growth in the second quarter that it will announce in two weeks. 

Netflix has become a global juggernaut, and not just because its market cap surpassed most of the traditional media networks through this year's torrid run. More than half of its streaming revenue is now being generated internationally where the ceiling is far higher than its more mature home market. 

Analysts can't seem to keep up. Earlier this week, we saw John Blackledge at Cowen boost his price target on the shares from $375 to $430. His firm's survey shows continued engagement in the platform, and he's boosting his long-term forecast for international subscriptions. Netflix's fundamentals keep improving with every passing quarter, and investors are more than happy to be going along for the ride. 

Carvana: Up 118%

We may not seem to be the in the golden age of auto retailing, but Carvana's connecting with shoppers hungry for a bit more buzz when they're looking for a used car. The larger-than-life multistory auto vending machines -- a feature at a dozen of its nearly 60 markets -- generate exciting eye candy that translates into free publicity for a company that's doing a great job of bringing a healthy dose of whimsy and convenience to the car-hunting experience. 

Revenue popped 122% in its latest quarter, mostly the handiwork of frenetic expansion. Carvana expects to provide coverage to 57% of the country by the end of the year. It's on track to deliver its fifth year in a row of triple-digit growth. 

Twilio: Up 138%

Netflix was already coming off a strong 2017, but sometimes a big year is largely about where we draw the starting line. Twilio shares declined last year, falling sharply off the peak levels set in late 2016. There were concerns last year that the in-app communications specialist was losing some of its biggest developers as Uber and Lyft began kicking the tires of rival services, and decelerating growth shook out some of its investors. 

Twilio's been on the comeback trail in 2018. Revenue soared 48% in its first quarter, well ahead of both what Wall Street pros were targeting and the 32% to 34% uptick it was originally modeling. Twilio has seen it active customer count grow by 33% over the past year, and with revenue growing even faster, it means that developers are leaning more on Twilio than ever to help their app users communicate within the mobile applications. Its dollar-based net expansion rate continues to clock in north of 100%. Armed with its best-in-class niche and a knack for conservative guidance that it can beef up as the year plays out, last year's laggard is this year's rock star. 

Netflix, Carvana, and Twilio all boosted their guidance after blowout first quarters, but these monster gains in 2018 are about more than just a single financial update. The companies are gaining market share. They are earning their momentum, and that's a good thing to note as we dive into the second half of the year.

Rick Munarriz owns shares of Netflix. The Motley Fool owns shares of and recommends Netflix and Twilio. The Motley Fool has a disclosure policy.