It was reportedly the biggest one-day loss in market value by any company in USA stock market history.
Other big Facebook investors also took a hit as disappointing second-quarter results obliterated $119 billion of market value, the most ever in a day for a USA company.
The lawsuit seeks class-action status and unspecified damages. Unfortunately, Facebook's spokeswoman declined to release a further comment about the ongoing complaint. "It's the F in FAANG, but what's to say that, 10 years from now, Facebook isn't the next Myspace and something else has taken its place?"
Both the slower growth forecast and heavier spending reflect problems largely of Facebook's own making.
Shareholders often sue companies in the United States after unexpected stock price declines, especially if the loss of wealth is large.
The Cambridge Analytica scandal forced the company to be more conscious of its privacy policies and allow users more control over their data. The drop also affected the net worth of the founder and the largest shareholder of the company - Mark Zuckerberg, which decreased by $15 billion and now stands at $67 billion.More news: What does Mueller have? Manafort trial offers glimpse
Currently, its stocks are at US$174.89 down 0.78 per cent.
The American technology entrepreneur, according to Forbes, is now worth $77.6 billion.
The revelation of a massive data leak in March was another factor that hurt the USA social networking giant.
More worrisome was the number of daily active users, which grew by just 22 million - the lowest such number since at least 2011.
The S&P 500 technology index fell 2.0 per cent, the most among the major S&P sectors. According to a report by Bloomberg, the 24 percent drop in Facebook's share valuation in after-hours trading is likely to be replicated in the normal trading session, which will cause Facebook's market capitalization to take a massive hit.