SAN FRANCISCO (Reuters) - Facebook FB.O, Amazon.com AMZN.O, Netflix NFLX.O and Google-parent Alphabet GOOGL.O were at the center of a volatile session on the U.S. stock market on Monday, cleaving $200 billion off of the so-called FANG group's combined market capitalization in two sessions.

FILE PHOTO: Facebook, Amazon, Netflix and Google logos are seen in this combination photo from Reuters files. REUTERS/File Photos/File Photo

With each company’s share price down between 14 percent and 24 percent in October, the quartet of stocks that has been the most popular trade on Wall Street in recent years appears to be in trouble.

The combined companies’ market capitalization has fallen from a record $2.5 trillion in July to $1.93 trillion on Monday.

The most recent catalyst hurting FANG was Amazon’s quarterly results late on Thursday, which missed analysts’ expectations and ignited worries the tech darling is finally facing stronger competition.

Amazon's stock has fallen 14 percent since then, its worst two-day decline since 2014, with the online retailer and cloud computing seller relinquishing its spot as the second largest U.S. company by stock market valuation to Microsoft MSFT.O.

That steep two-day drop in Amazon and losses in other FANG stocks has reduced the group’s market capitalization by $200 billion to $1.93 trillion.

The next catalyst for FANG may come on Tuesday, when Facebook posts its quarterly results after the bell. Analysts on average expect the social network’s revenue to have grown 33 percent in the third quarter, its slowest quarterly growth rate since the company listed its shares in 2012, according to Refinitiv.

Investors in recent months have become increasingly concerned about rising costs and damage to Facebook’s reputation following criticism of its management of users’ personal data.