Stocks as a whole appear set to post modest losses for 2018. The S&P 500 is down 2% with just a few trading days left, which many investors consider a disappointment given that the index was up 8% through the first nine months of the year.

But while stock prices have floundered lately, many businesses are enjoying head-turning sales growth. Below, we'll look at three companies, Facebook (NASDAQ:FB), Wayfair (NYSE:W), and Netflix (NASDAQ:NFLX), that are among the S&P 500's fastest-growing stocks for the year.

Staying social

Facebook shares have taken a hit this year. The 20% haircut looks even worse when compared with peer Twitter, which is up nearly 40% in 2018. But investors can't complain about the social media titan's revenue position. Sales are up 40% through the first three quarters of the year, after all, compared to Twitter's 25% increase.

Four adults using smartphones.

Image source: Getty Images.

Facebook is benefiting from healthy user trends, with its monthly active user base rising 10% in the most recent quarter as Twitter's base declined. Advertisers are paying premium rates to gain access to those feed scrollers. In fact, the company's average revenue per user, a key profitability metric, jumped to $6.09 from $5.07 a year earlier.

A smaller portion of those profits is flowing through to the bottom line due to aggressive spending on the part of the management team, and those investments help explain why operating margin had slumped to 42% of sales from 50%. Facebook's brand has also been challenged lately, especially by data privacy concerns. These issues, plus the company's huge revenue base, should combine to slow its growth pace significantly in 2019. But, with $15 billion of net income booked through the first three quarters, Facebook remains one of the market's strongest businesses.

A flexible selling platform

Wayfair's revenue is up 45% over the last nine months, which puts it well ahead of Amazon.com's 37% increase. The home furnishings e-commerce retailer now counts over 13 million customers, up from 10 million a year ago, and repeat business from these satisfied clients continues to grow as a percentage of order volume.

The management team believes that, bolstered by positive engagement trends like these, Wayfair can continue to grow at a faster rate than the broader home furnishings niche thanks to its online focus and its ability to leverage its platform into new niches. The early results from this past holiday shopping season seem to support that thesis, as sales growth jumped to a 58% pace around Black Friday, compared to 53% in the prior year.

There's no denying Wayfair has solid momentum heading into 2019, but investors are eager to see some evidence of a march toward profitability considering the company is on pace to post its fifth consecutive year of net losses.

Streaming into the record books

Its stock is sitting well below the highs it set in the summer months, but Netflix is still one of the market's best performers this year. It's not hard to see why, either.

The streaming-video giant has ridden a surge of subscriber gains, plus rising average monthly fees, to a 38% revenue spike so far in 2018. Operating income has more than doubled in that period, reaching $1.4 billion compared to $593 million a year earlier.

A woman watches TV on the couch.

Image source: Getty Images.

CEO Reed Hastings and his team believe they'll add about 9 million members during the final quarter of the year to push global subscribers to nearly 150 million. Netflix started 2018 with 117 million streaming users.

On the downside, its spending on original content promises to keep cash flow in negative territory for at least the next few years. However, just like Wayfair and Facebook, Netflix is betting that investments it makes today will support many more years of growth at the expense of a temporary hit to profits. There are favorable odds to that bet, considering that exact approach has helped it expand much faster than most of its S&P 500 peers in 2018.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Demitrios Kalogeropoulos owns shares of Amazon, Facebook, and Netflix. The Motley Fool owns shares of and recommends Amazon, Facebook, Netflix, Twitter, and Wayfair. The Motley Fool has a disclosure policy.