For the past 20 years, The Digital Millennium Copyright Act (DMCA) has protected U.S.-based firms from the consequence of their users' actions. (The EU has a similar directive.) As a result, social media firms can't be sued or prosecuted, regardless of what their users upload and post.
However, a recently-passed law just changed all of that. It's called the "Allow States and Victims to Fight Online Sex Trafficking Act" (aka FOSTA) and it makes website operators liable for any user-generated content that "promotes or facilitates prostitution."
In other words, Internet providers can now be arrested for user-created content that might result or seems to result in illegal behaviors. Or legal behaviors, if you live in most parts of Nevada.
FOSTA creates a legal precedent to hold Internet providers responsible for user-created content that drives other behaviors. Hate speech might lead to murder and terrorism, for instance. Therefore, it's easy to imagine that the US government will pass laws similar to FOSTA holding Internet providers legally liable for that content. Other examples of user-content that might face FOSTA-style laws include sexual harassment, racism, fake news, and election interference.
The possibility--nay, the inevitability--that the government will take that route is freaking out the investment community.
According to information provided to me by the investment firm Arjuna Capital and The New York State Common Retirement Fund (the third largest public pension fund in the US), investors plan to challenge Twitter, Facebook, and Alphabet, Inc. (Google) at their upcoming annual meetings and demand
"detailed reports on the scope of platform abuses, and full disclosure of practices to curb risks posed by recent content management controversies [because] industry leaders are being held increasingly accountable by lawmakers on key issues such as election interference, fake news, and hate speech [and] all these issues can have a direct impact on companies' finances, operations and reputation."
In other words, investors are telling the big three that if don't clean up their act, the US government will pdo it for them. And because no companies or investors want to see CEOs in handcuffs, investors are in essence demanding that the big three censor user-created content... before that content becomes a huge legal liability.
And this, for all intents and purposes, means the end of free speech on the Web, regardless of if it's due to self-censorship or government mandated censorship.