Dating apps are two-sided marketplaces, and the product is other people. Liquidity measures how full that marketplace is for a typical member: enough people within the right distance, age range and preferences, who are active and likely to reply.
Low liquidity shows up as empty or repetitive discovery screens, few replies and high uninstall rates. It hits small towns, niche apps and new countries first.
Apps manage liquidity by widening search radius, launching city by city, focusing marketing on dense areas, or filling gaps from a shared member network (backfill).