The rapid adoption of commercial Generative Artificial Intelligence (Gen AI) products raises important questions around the impact this technology will have on our communicative interactions. This paper provides an analysis of some of the potential implications that Artificial Intelligence-Mediated Communication (AI-MC) may have on epistemic trust in online communications, specifically on social media. We argue that AI-MC poses a risk to epistemic trust being diminished in online communications on both normative and descriptive grounds. Descriptively, AI-MC seems to (roughly) lower levels (...) of epistemic trust. Normatively, we argue that this brings about the following dilemma. On the one hand, there are at least some instances where we should epistemically trust AI-MC less, and therefore the reduction in epistemic trust is justified in these instances. On the other hand, there are also instances where we epistemically trust AI-MC less, but this reduction in epistemic trust is not justified, resulting in discrimination and epistemic injustice in these instances. The difficulty in knowing which of these two groups any instance of AI-MC belongs to brings about the AI-MC dilemma: We must choose between maintaining normal levels of epistemic trust and risking epistemic gullibility when reduced trust is justified, or adopting generally reduced epistemic trust and risking epistemic injustice when such reduced trust is unjustified. Navigating this choice between problematic alternatives creates a significant challenge for social media as an epistemic environment. (shrink)