Bets against popular tech stocks netted investors over one billion dollars in profit last week, according to data from S3 Partners.

FAANG stocks, Facebook Inc. (FB), Amazon.com Inc. (AMZN), Apple Inc. (AAPL), Netflix Inc. (NFLX) and Google's parent company, Alphabet Inc. (GOOGL), were among the biggest victims of last week’s equity sell-off, falling 4.7% and 3.6% on Friday alone after Amazon and Alphabet's third-quarter results fell short of expectations. The upshot was $1.62 billion in mark-to-market profits for the growing number of investors who saw this coming.

The strong momentum of FAANG stocks in recent years has led some analysts to question their valuations, particularly as global growth slows and monetary policies tighten. Data from S3 indicates that investors have been taking these concerns seriously.

Short interest in the five big tech stocks surged 7% throughout October, S3 reported, making them among the 10 most shorted companies on the U.S. stock market. During the month, the number of Facebook, Amazon, Apple, Netflix and Alphabet shares shorted increased by 2.9 million, 1.5 million, 1.6 million, 419 thousand and 58 thousand, respectively, bringing total short interest in FAANG stocks up to $30.5 billion.

As of Oct. 29, Apple led the way as the most shorted FAANG stock. Traders have betted $9 billion against the iPhone maker, according to S3 data, making it the second most shorted U.S. stock after Tesla Inc. (TSLA). Amazon came in third, followed by Qualcomm Inc. (QCOM), Starbucks Corp. (SBUX), Netflix, Microsoft Corp. (MSFT), Facebook, Alphabet and CVS Health Corp. (CVS).

Source: S3 Partners

Ihor Dusaniwsky, managing director at S3 Partners, said betting against FAANG stocks generated higher returns than shorting the S&P 500 over the past week. While the broader market declined 3.9%, FAANG stocks fell -4.7%, he added, indicating 18% outperformance.

Based on this success, and concerns about the direction of corporate profits, Dusaniwsky predicted that FAANG stocks will continue to be punished by short-sellers. However, he also pointed out that a sharp increase in bets against these once-beloved companies could eventually see them fall back into favor with investors.

"October's significant increase in shares shorted will increase the potential for some volatile swings to the upside," he wrote.