There isn't anything unique about your account on most social media platforms. This isn't a "plant your flag" situation like when trying to prevent identity theft. You don't need to register your account before a bad actor does. Sure, I created an online account with the IRS, credit bureaus, etc. before somebody else could. That's important because they are tied to unique identifiers like your SSN, etc. But somebody could just create a social media account impersonating you even if you already have an account on that social network. There isn't anything enforcing the uniqueness.