Stock Market Pixabay
Image source: Pixabay.

Following the S&P 500's (SNPINDEX:^GSPC) latest 50-point tumble over a two-day period, the broad-based index that is arguably the best representation of the health of the U.S. economy and stock market is now down for the year, albeit marginally.

Over the course of 2015, the S&P 500 has vacillated wildly between being up and down for the year, demonstrating that Wall Street and investors are clearly indecisive as to what should happen next with the U.S. economy and stock market.

The S&P 500's best stocks in 2015
Within the S&P 500, which is comprised of slightly more than 500 of the largest companies in America, some stocks ascended to the heavens. In fact, through Dec. 3, 2015 there were two stocks that have more than doubled year to date. The big question is, did these companies exhaust their momentum in 2015, or is there a solid enough growth story behind these companies to propel their stock prices higher in the future? Let's have a look.

Netflix (NASDAQ:NFLX): Up 164%
After turning in a miserable year in 2011 that saw its shares sink by well over 50%, streaming-content provider Netflix has been virtually unstoppable since. This year alone, Netflix's market valuation has ballooned from around $21 billion to more than $54 billion.

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Image source: Netflix.

The major catalyst for Netflix has been its ongoing push into streaming, especially in international markets. Wall Street and investors have come to terms with the fact that pushing into international markets is a costly venture -- as are licensing fees for that matter -- that isn't likely to immediately pay off. However, long-term investors also realize that international subscribers are the key to Netflix's growth, because international markets are largely untapped.

For example, international streaming resulted in a $68 million loss in the third quarter, and is expected to generate $117 million in losses in Q4 despite steady expected revenue growth of around 10% from the sequential third quarter. More importantly, Netflix's net subscriber additions are accelerating with each passing quarter.

Netflix forecasts 29.49 million total memberships internationally by the end of the year. Over time, as costs in international market lessen, Netflix should begin to reap margins that are similar to what it earns on its U.S. members, which ranges between 14% and 18%. 

Netflix is also benefiting from its partnership with Disney, which was inked back in 2012. The Disney deal prevented new movies from streaming on Netflix until 2016; but with the calendar about to change to a new year, and Disney set to release what could be the highest-grossing film of all time, Star Wars: The Force Awakens, Netflix could be sitting pretty. This deal could be critical to drawing in entire households as it appeals to both parents and their children.

Can the good times continue? A lot will depend on the forgiving nature of investors, considering that Netflix is trading at a premium forward P/E of nearly 500, and whether or not streaming rivals can ink licensing deals that pull on-the-fence streaming customers away from Netflix. With a burgeoning lineup of exclusive series of its own, Netflix appears set for another good year in 2016; although, I would suggest tempering your return expectations a bit moving forward until we see an operating improvement in its overseas-streaming segment.

Amazon.com (NASDAQ:AMZN): Up 118%
Online retail giant Amazon.com is the other half of 2015's amazing duo that's bolted higher by more than 100%. Amazingly, it's Amazon's quarterly earnings reports, which have often been a sore spot for skeptics, that have provided nearly all of the upside spark year to date.

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Image source: Amazon.com.

For more than a decade, Amazon.com's strategy has been to forgo boosting its profits, and instead, reinvest as much of its operating cash flow as possible back into its online business and satellite operations. For investors willing to look past EPS as the only determinant of business health, Amazon has been growing at a phenomenal pace.

In the third quarter, Amazon announced that its trailing 12-month operating cash flow had improved by 72%, to $9.8 billion, with quarterly revenue rising 23% in Q3 2015 from the year-ago period. Things look even better on an operational basis -- 30% year-over-year growth -- if adverse foreign currency effects are removed. Part of this can be attributed to the 230% year-over-year increase in Amazon's active customers, and its focus on Prime, which keeps members loyal to the service.

However, a big growth driver of late has been Amazon's Web Services, or AWS. Despite accounting for less than 10% of total revenue, AWS accounted for more than half of Amazon's third-quarter profit.

Also, per Foolish colleague Jamal Carnette, the company's AWS is on pace for about $7.3 billion in extrapolated annual sales -- an 81% year-over-year increase -- and now counts around 1 million customers. Emphasizing AWS is a smart move for Amazon because it offers substantially higher margin potential than its retail operations. 

Can Amazon.com push even higher in 2016? It's a tough call considering Amazon's $312 billion valuation and forward P/E of 117. Like Netflix, investors have been willing to overlook its lack of profitability, and have instead focused on Amazon's cash flow and top-line growth rate. However, a valuation north of $300 billion pushes the envelope of how much Wall Street and investors can ignore before they demand substantial profits.

In my best guess, Amazon is going to need to trounce the Street's earnings forecasts next year if it hopes to maintain or grow its current valuation. However, I'd also suggest that investors refrain from giving too much credence to the company's quarterly reports, and remain focused on its long-term outlook.

Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

The Motley Fool owns shares of and recommends Amazon.com, Netflix, and Walt Disney. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.