The dissertation consists of three essays in information economics. In the first chapter, "Social Learning in General Information Environments," I consider a variant of the model of herd behavior introduced by Banerjee (1992) and Bikhchandani, Hirshleifer, and Welch (1992). I study a social learning model in which agents make decisions sequentially and learn about an unknown payoff-relevant state through two sources – a signal about the state itself (a state-signal) and a signal about the actions taken by previous agents (an action-signal). The objective is to provide general conditions on the action-signals that lead the agents to eventually behave as if they know the state, i.e., that lead to information aggregation. When the agents’ action-signals are what I call weakly separating, it is shown that information aggregation occurs when the agents’ state-signals are unboundedly informative in the sense of Smith and Sorensen (2000). This result provides a unifying criterion to evaluate when information aggregation occurs. The theory is illustrated with applications to privacy protection on digital platforms and the analysis of robustness of information aggregation to third-party manipulation. The second chapter is entitled "Platform Design for Costly Learning''. This chapter studies the optimal design of information disclosure on a platform to incentivize its users to collectively acquire costly information about the quality of a product. A constant flow of users arrive in sequential order. Each user observes information disclosed by the platform and may acquire a costly private signal about the product quality before making his purchase decision. Within a large class of environments, I show that it is optimal for the platform to release no information early on to induce user exploration, and, at a later point in time, publish a list of potentially good products once and for all. Compared to an environment without intervention, the optimal design induces a Pareto improvement for the consumers and the platform. Finally, in the third chapter, "Monopoly Pricing with Endogenous Information Response," I study how endogenous informational responses of third parties affect consumer and producer surpluses in a monopoly pricing setting. The analysis is motivated by the fact that rapid developments in digital technology have given consumers access to new information sources that allow them to learn about a product prior to making purchase decisions. These media sources are typically controlled by third parties (e.g., IMDb in the movie industry). To understand these phenomenon, I study a mechanism design problem for a seller who posts a selling mechanism to sell a product to a consumer. Unlike the standard mechanism design paradigm, a third party observes the seller’s choice of selling mechanism and then designs the information structure of a signal about the product quality. The consumer observes the signal realization as well as the information structure, and then chooses an action in the selling mechanism. It is shown that the seller cannot benefit from designing a mechanism that directly punishes the third party. Equilibrium selling mechanisms and signal structures are also studied for the case when the third party cares about a weighted sum of the buyer's and the seller's surpluses.