This paper examines a digital platform issuing proprietary money to compete against a legacy fiat marketplace. Platform money kickstarts two-sided network effects, complementing matching services and strengthening market power. The platform's inflation markup over the Friedman rule is disciplined internally by demand elasticity and externally by legacy inflation. Platform money improves welfare when the issuer possesses superior matching technology. Competitive public currencies (e.g., CBDCs) structurally constrain platform monopoly power. Finally, platforms may optimally reject outside money to strategically segment the user base, softening competition to extract higher seigniorage.