SAN FRANCISCO >> Investors for years have seemingly adored technology stocks as much as most people love their smartphones.

But Wall Street has suddenly soured on Silicon Valley and the rest of tech, triggering a stomach-churning downturn in a turbulent October.

Some of the hardest hit stocks belong to five companies — Facebook, Apple, Amazon, Netflix and Google. They have collectively attracted billions to their products, carving out lucrative markets they each dominate in an increasingly digital world. Investors latched on to their success and gave them their own acronym, “FAANG.” (It’s still in use even though Google now trades under the stock of its parent Alphabet Inc.)

What a difference a month makes. Since the end of September, individual FAANG stocks have plunged between 4 percent and 20 percent, collectively wiping out nearly $400 billion in paper shareholder wealth.

The downturn may seem puzzling, given that Apple’s iPhone sales are booming, the online shopping traffic keeps sending more consumers to Amazon, people are constantly asking Google to enlighten and direct them, people keep posting on Facebook and Netflix has never been a more popular entertainment destination.

But these companies are facing rising challenges. President Donald Trump has escalated a trade war with China, for instance, and governments are starting to consider tougher regulation that could curb tech’s influence. Employees at some large tech concerns are increasingly restive about their companies’ contributions to military and immigration-related projects.

Much of that contributes to concerns that the tech companies won’t be growing as much and as quickly as investors had expected. “We are starting to see ‘fork-in-the-road’ situation for technology,” said Wedbush Securities analysts Daniel Ives.

Investors currently are betting it will be a bumpy road. The tech-driven Nasdaq index is 12 percent below the high it reached in August.

The big-name tech stocks have been faring so well for so long that investors have been betting on even bigger things to come from the companies. Those wagers might take longer to pay off, or worse, fizzle completely if a slowing economy or a recession undermines their future growth.

Facebook and Google, for instance, might not be able to entice as many new users to their free digital services, and the advertising that generates most of their revenue might shrivel away.

For Amazon, it might mean consumers curtail their spending on merchandise in its e-commerce site or decide they really don’t need an internet-connected speaker like the Echo after all. Netflix might have more difficulty attracting subscribers, and could even start seeing more cancellations if households feel squeezed.

Rising interest rates are also weighing on stock prices, analysts say. Higher rates reduce the present value of future corporate earnings, which in turn undermines the justification for the lofty valuations — and high share prices — of tech companies.

These valuations are commonly measured by price-to-earnings ratios — the amount investors are willing to pay for each dollar of anticipated earnings. Consider Netflix, a company that began renting DVDs through the mail during the late 1990s, and which not long ago was considered to be worth more than Walt Disney Co. and its Magic Kingdom.

Even after the recent sell-off, Netflix’s price-to-earnings ratio stands at $107 for every $1 in earnings. By comparison, Disney’s is a more reasonable $14 for every $1 in earnings — and it’s also now worth about $37 billion more than Netflix.

The long tech rally boosted two members of the FAANG club — Apple and Amazon — to trillion-dollar market valuations, making them the first U.S. companies to reach that milestone.

But Amazon’s market value now stands below $800 billion. Apple could also be knocked out of the $1 trillion club if its earnings for the latest quarter disappoint investors the same way Amazon and Alphabet reports did this past week.

“There are a lot of white knuckles out there right now, so all eyes are on Apple to emerge as the knight in shining armor,” Ives said.

Apple’s report for its fiscal fourth quarter is scheduled to be released Nov. 1. Analysts expect Apple to have earned $13.5 billion for the July-September quarter, or about $6 million every hour.