Earlier quoted context omitted.
You don't announce the discovery of a miracle cure via social media, you announce it via clinical trial papers.
the medium you use does not establish the truth of your claim.
(That it often fails at this isn't a point in dispute. The fact that the medium entails a process and that the process, at least nominally, serves truth is.)
There's also the reputational role that such journals serve, effectively transferring the trust bestowed on the journal to the authors appearing within it, where trust is a belief extended beyond the extent of verifiable fact though not in opposition to them (as in the case of blind faith).
To that extent, and in a world in which each individual receiving a piece of information cannot independently assess and verify that information, your premise is in large part false: the medium used does serve to indicate the truth of a claim.
(As someone who's made a point of publishing pseudonymousely and in numerous online, unreviewed media, I'm aware of the challenges of trying to assert facts, even those which are reasonably independently verifiable, without the benefits of a persistent and highly-established identity or other indicia of trust, reputation, or credentialing. In balance I prefer the freedoms that come with this, though the challenges are also considerable.)