N. Wernerfelt is the first author and authors are listed in reverse alphabetical order. The authors thank participants in the 13th Workshop on the Economics of Advertising and Marketing, the 2022 INFORMS Society for Marketing Science Conference, the 2023 Marketing Science Institute Summit, the 2023 Social Science Research Council Workshop on the Economics of Social Media, the 16th Annual Federal Trade Commission Microeconomics Conference, the 2024 Allied Social Science Association meetings, Brookings, and the Stanford Digital Economy Laboratory, as well as seminar participants at Carnegie Mellon, Duke, Harvard, London Business School, Massachusetts Institute of Technology, National University of Singapore, New York University, Northwestern, Rochester, Toronto, University College London, University of British Columbia, University of California Berkeley, University of California Los Angeles, University of California San Diego, University of Chicago, Wharton, and Yale. The authors are especially grateful to Isaiah Andrews, Jean-Pierre Dubé, Sam Goldberg, Günter Hitsch, Nien-hê Hsieh, Garrett Johnson, Ilya Morozov, Fiona Scott Morton, Stephan Seiler, and Duncan Simester, who each provided valuable feedback. All errors remain our own.