Sorting out which claim it is
The legal theory depends on what was actually posted. False statements of fact that damage your reputation raise defamation questions, while opinion, however harsh, usually does not. Photos, videos, and writing copied without permission are usually copyright matters. Using your name or likeness to sell something can raise right of publicity issues, and an account posing as your business may raise trademark concerns. Each theory has its own rules, defenses, and filing deadlines, and some of those deadlines are short. New York's anti-SLAPP law can leave a plaintiff paying the other side's legal fees when a claim targets speech on a public issue and lacks a substantial basis, so a speech case deserves a careful look before it is filed.
The platform is usually not the defendant
Federal law gives social media companies broad protection from claims based on what their users post, so the usual target is the person who posted rather than the service. That protection generally does not reach intellectual property claims, which is one reason copyright takedown procedures matter so much online. When the poster is anonymous, it may be possible to ask a court to order the platform to identify the account holder, and courts generally require a real showing before they unmask a speaker. Separately, there is large litigation against platforms over how their products affect young users; those cases rest on different theories and are a different kind of matter altogether.
What to preserve before it disappears
Posts get deleted, accounts go private, and stories vanish on their own schedule. Capture full screenshots showing the account name, the date, and the page address, and save links to comments and shares that show how far the content spread. Keep records of any harm you can trace, such as lost clients, canceled contracts, or messages from people who saw it. Do not reply publicly or create accounts to argue back, because those exchanges become part of the record too. If you are the one accused over a post, talk to a lawyer before deleting it, since deletion can create its own problems once a dispute is underway. In a first conversation we identify which claim, if any, fits, and whether a takedown request, a letter, or a lawsuit is the proportionate step.
