It was a wild week in tech as a range of popular tech companies reported earnings. But three in particular -- Facebook (NASDAQ:FB), (NASDAQ:AMZN), and Twitter (NYSE:TWTR) -- were the most intriguing.

  1. Shares of Facebook fell about 20% after management forecast decelerating revenue growth and rising costs.
  2. Amazon managed to impress investors with surging profits.
  3. Twitter stock sank when the company missed the mark on monthly active users.

Here's what investors should know.

Facebook CEO Mark Zuckerberg presents 10-year plan at F8 conference in 2016

Facebook CEO Mark Zuckerberg. Image source: Facebook.

Facebook's growth slows

A glance at Facebook's financial results, featuring 42% year-over-year revenue growth and a 32% increase in earnings per share, might lead many investors to believe things couldn't have gone better. But some other details help shed light on why the Street was disappointed.

First, though Facebook's revenue growth was strong, it was down from 49% year-over-year growth in Q1 and below the consensus analyst estimate for the metric. Facebook's second-quarter revenue was $13.2 billion, while analysts' consensus forecast called for revenue of $13.4 billion. 

Second, management forecast a further deceleration in revenue growth even as it expected its operating expenses to rise sharply. In the coming quarters, year-over-year revenue growth rates will "decline by high-single-digit percentages from prior quarters sequentially in both Q3 and Q4," management noted in its second-quarter earnings call. Furthermore, with expenses expected to rise 50% to 60% year over year in 2018 and with total expenses expected to rise faster than revenue in 2019 as well, management forecast its operating margin to fall from 44% today "toward the mid-30s on a percentage basis" when looking beyond 2018. 

Amazon's profits soar

Amazon's kept up its strong growth in Q2, with revenue rising 39% year over year and earnings per share skyrocketing from $0.40 in the year-ago quarter to $5.07 -- more than doubling the consensus estimate for EPS. In addition, Amazon's operating income of $3 billion crushed management's guidance for second-quarter operating income to be between $1.1 billion and $1.9 billion. 

Amazon's significant outperformance in regard to profitability was fueled by the company's highest operating margin ever of 5.6% -- up about 400 basis points year over year.

In addition, Amazon's cloud computing business, Amazon Web Services, saw accelerating year-over-year revenue growth for a third quarter in a row.

Twitter's monthly active users decline

Despite revenue that was above the consensus analyst estimate and earnings per share that matched the consensus forecast, Twitter shares were slammed after the earnings report Friday morning. The sell-off was likely due to Twitter's sequential decline in monthly active users. Monthly active users for the quarter were 335 million, down from 336 million in Q1.

The primary reason for the pullback in users was the company's efforts to delete suspicious and spammy accounts. In the quarter ahead, Twitter expects continued efforts to clean up its accounts to put more pressure on user metrics. "Based on our current level of visibility, we expect the decline to be mid-single-digit millions" of monthly active users, management said in Twitter's second-quarter shareholder letter.

Beyond this negative trend, Twitter's results were solid. Revenue growth accelerated to 24% on a year-over-year basis and earnings per share increased from a loss per share of $0.16 in the year-ago quarter to $0.13.