We study how content moderation facilitates communication on online platforms. A sender transmits information to a receiver, exerting effort to signal their truthfulness. Communication fails without moderation because the effort required is prohibitive. Moderation resolves this problem by making effort a more powerful signal of veracity. However, moderation crowds-out sender effort, decreasing content quality on the platform. A socially optimal or profit-maximizing policy may therefore involve limited moderation. We study the choice between being a platform or broadcaster, how moderation influences competition for attention, and the effects of misinformation actors, AI-generated content, and moderator errors on the sustainability of communication.