Image source: Fitbit.

2016 has been a pretty great year for investors. After rising 6%, the S&P 500 is trading near an all-time high.

But not every stock has performed well. Shares of Fitbit (FIT), Twitter (TWTR), and FireEye (MNDT) have all shed at least 20% of their value in 2016, devastating shareholders in the proces. The declines have been warranted -- each company has turned in a series of somewhat disappointing earnings reports. But all three companies remain intriguing, and at current levels, they could offer investors interesting opportunities.

Still growing revenue at a rapid pace

Shares of cyber security vendor FireEye tumbled in May after the company turned in a first-quarter earnings report that fell short of analyst projections. On an annual basis, FireEye's revenue rose 34%, but that was less than the 37% Wall Street had been expecting. FireEye made its guidance, but its numbers came in at the lower end of its range, which, for a fast-growing company like FireEye, is often viewed as a disappointment. At the same time, CEO Dave DeWalt announced his departure, dashing hopes that FireEye could be acquired.

At current levels, FireEye is trading near an all-time low, which could offer an opportunity for investors. Admittedly, those investors should approach FireEye with a high degree of risk tolerance: The company isn't profitable, and doesn't expect to be anytime in the near future. But its business remains interesting, as it dominates the market for advanced persistent threat detection.

In the past, FireEye shares have surged in the wake of high-profile hacks. FireEye's technology attempts to counter threats from increasingly sophisticated hackers, including those operating at the behest of foreign governments. Recent headlines about potential Russian meddling in the U.S. presidential election only serve to reinforce the necessity of such safeguards.

FireEye's most recent earnings report wasn't as bad as it may seem, either. Although its revenue fell short, its billings growth met expectations. FireEye's business is shifting away from the sale of physical appliances, which puts pressure on its top line growth, but create the opportunity for long-term margin improvement. New CEO Kevin Mandia has a long history in the industry, and could help the company outperform and reset investor expectations.

Leading the market for wearables

Fitbit, in contrast, is profitable. In fact, for a relatively young publicly traded company, Fitbit trades with a reasonable trailing price-to-earnings ratio around 23.

Fitbit's lineup of bands and smartwatches, including its recently released Alta and Blaze, attempt to track and categorize the activity of their users. In many cases, the company has succeeded. Fitbit leads the market for wearables, according to research firm IDC. It captured 24.5% of the market last quarter, ahead of Chinese handset giant Xiaomi (19%) and Apple (7.5%), as its unit shipments rose more than 25% on an annual basis.

Throughout 2016, Fitbit has been plagued by disappointing earnings reports and persistent fears that demand for its products could be devastated by increasingly competent smartwatch rivals. In May, Fitbit shares plunged after the company reported a quarter that was better than expected, but gave disappointing guidance for the rest of the year. Still, it's projecting revenue growth of around 44%.

The smartwatch menace could be overstated. Earlier this month, IDC noted that in the second quarter, demand for the Apple Watch declined 55% on an annual basis. The Watch is more than a year old, and a second generation is likely coming, perhaps as soon as September. But for such a new and innovative product category, a drastic decline is disheartening.

If Fitbit remains resolute in the face of an increasingly competitive landscape, and continues to drive revenue growth at such a rapid pace, investors who bought in near the stock's all-time low could be rewarded.

Still looking for a turnaround

Twitter is also trading near an all-time low in the wake of a disappointing earnings report. Shares of the social media giant are down more than 30% this year, with much of that loss coming within the last few days. Twitter's revenue rose 20% on annual basis, but that was worse than what analysts had anticipated, and was down sharply from the 60% revenue growth the company reported in the same quarter last year. In total, Twitter's monthly active users rose just 3%.

Still, Twitter has more than 300 million active users. That's an impressive figure, and it could monetize its users to a greater extent if it can improve its product. CEO Jack Dorsey is attempting to turn the company around, but hasn't had much time, as he's been at the helm for just over a year (and as permanent CEO for less than a year). Twitter is expanding its efforts in live video streaming, which could drive revenue growth in the future. Meanwhile, a recent string of major tech acquisitions has strengthened the possibility that Twitter could eventually be acquired.

Twitter has been a disappointment for investors since it entered the public markets back in 2013. It hasn't been able to capitalize on its user base as much as other social networks. But Twitter users remain loyal and engaged. If Twitter can fix its problems, it could reward shareholders at some point in the future.