The United States Federal Trade Commission has demanded Facebook, TikTok, Twitter and several other widely-used social media companies reveal exactly how they use their user's data.
Facebook-owned Whatsapp, Amazon, ByteDance (TikTok), Discord, Reddit, Snapchat and Google's YouTube have also been recipients of requests for more information on their data practises.
The FTC is keen to expand their understanding of how these social media sites collect and compile demographic information and what happens to users' personal information is manipulated by algorithms.
The FTC wants to understand how business models influence what Americans hear and see.
Compiling more information on exactly how these sites affect children and teenagers is also top of the agenda.
A decisive 4-1 vote ensured the investigation was given the green light, but Noah Joshua Philips, who voted against the action, suggested the inquiry would waste the commission's resources due to its overly broad scope.
All parties will have 45 days to respond to the FTC's order, which could result in regulatory action further down the line.
"Never before has there been an industry capable of surveilling and monetizing so much of our personal lives," FTC commissioners Rohit Chopra, Christine Wilson and Rebecca Slaughter wrote in a joint statement published on Monday.
Social media and video streaming companies now follow users everywhere through apps on their always-present mobile devices. This constant access allows these firms to monitor where users go, the people with whom they interact, and what they are doing ... too much about the industry remains dangerously opaque.
Elsewhere in the statement, the commission says they are particularly interested in "how children and families are targeted and categorized" and "whether we are being subjected to social engineering experiment."
How have the tech companies responded?
All nine companies have been contacted by journalists from major news outlets, although most have declined the opportunity to comment.
However, a Twitter spokesperson has said in a statement that the site is "working, as we always do, to ensure the FTC has the information it needs to understand how Twitter operates its services."
Discord, on the other hand, referenced its lack of ads and paid subscription service, saying the site "takes user privacy very seriously and we look forward to working with the FTC to answer their questions about our privacy practices."
Pressure mounting on Facebook
The news comes less than a week since the Federal Trade Commission announced it was suing Facebook along with more than 40 US states for "maintaining its personal social networking monopoly through a years-long course of anti-competitive conduct."
They allege Facebook used its dominance to take out rivals and create a market with less choice and fewer privacy protections.
The lawsuits will place particular focus on two purchases made by Facebook: the $1 billion acquisition of Instagram in 2012 and the $19 billion buy-up of Whatsapp in 2014.
Other issues include Facebook's apparent desire to block potentially competitive developers from using its systems and programmes.
YouTube owners Google are also facing another antitrust lawsuit, which is being filed against the company by the Department of Justice and more than ten states.