While current U.S. laws bar people from suing because of content posted by third parties, some accountability should be laid at the feet of developers who purposely manipulate the technology to put profit over the safety and well-being of its users. With that being said, you do have options to hold another individual accountable for harmful content. Here are some current legal angles under which a claim might be brought against another individual that posts on social media:
Social media defamation - There's no question that untrue statements about you or your business can cause harm. When someone posts a defamatory remark on social media, it's called libel. However, you may recognize it as cyberbullying or cyber harassment. Since social media is so prevalent and many people and businesses depend on it, any disparaging remark can have far-reaching consequences, including harm to mental health.
Still, there is little oversight on profiles created anonymously to damage others. While many people feel they can make defamatory statements based on freedom of speech, the First Amendment applies to freedom from government censorship. While people can say what they want, that doesn't mean there won't be consequences, especially if it's damaging and untrue. Still, an absolute defense to defamation is truth. That means if the person who made negative comments about you is true, it's unlikely you'd successfully bring a claim against them.
Here are two scenarios to illustrate this concept:
You own a restaurant, and a customer gets angry because they forgot to make a reservation for an important event. You weren't able to accommodate their last-minute request. In response, they post a fake picture of your signature dish with a used band-aid in it, claiming, "look what I just found in my food at XYZ restaurant." Suppose you can prove the customer never had that experience at your restaurant, and you suffered financial losses as a result of that post. In that case, you might have a clear case of libel that you could pursue.
Here's the second scenario:
You own a restaurant, and a food critic comes in and orders your signature dish. The critic then makes a post stating, "I had the worst spaghetti of my life tonight at XYZ restaurant. In the second scenario, it wouldn't be considered liable because it's based on their opinion. Only the food critic can know if it was indeed the worst spaghetti dish they've ever had. Although it can be hurtful, there's likely no path forward using the legal system.
Intellectual property claims - You have a cause of action if a social media site violates your intellectual property rights. That means the content you produce or post on the platform is yours. Suppose you're an artist concentrating on gothic themes, and you upload your latest painting of crows in a field. Since you're subscribed to groups that celebrate gothic themes, you run across another user who has posted your art as their own. In that case, you could take legal action against the social media platform hosting the content for violating your intellectual property rights.
Failure to warn - Perhaps the most interesting legal angle that could bring down social media giants is failure to warn. This is when social media developer conduct is responsible for harm to others. For example, in theory, you might be able to sue if a loved one became radicalized through manipulative algorithms that are designed to feed users content based on interests. While social media platforms are largely immune from legal action based on what third parties post, if they fail to self-monitor and violent content is pushed out (based on their algorithm) until your loved one's mental health deteriorates and leads to violent actions, the social media platform could be held accountable.