The United States (U.S.) is in the midst of a paradigm shift in who creates news. It is widely known that the internet has in the past few decades displaced television and print newspapers as the primary medium where news is consumed. However, the past two decades have also seen a shift in "who" is communicating the news. People consuming digital news, and especially young people, are now less likely than they were to get their news from journalists or television broadcasters, and more likely to receive it from a nebulous figure commonly referred to as the influencer. People are not only paying more attention to influencers, but there has seemingly been a remarkable growth in the type of and number of influencers in the last decade. This growth has been enabled both by the maturation of new, massively-networked social media platforms and the industrialization of influencing as a profession and an economic infrastructure. In this dissertation, I use quantitative and qualitative methods to analyze case studies of U.S. political discourse networks to better understand the structure, tactics, and consequences of our new era of influencer media. After conducting a literature review of previous work on influencers in political communication and media studies, I propose flexible ways to represent both influencers and what I call influencer communities in social media networks. Influencer communities are assemblages of influencers working cooperatively and antagonistically to earn the attention of networked audiences on large social media platforms. Via four case studies of Twitter discourse in the U.S., each of which presents a new method for understanding influencers' behaviors and organization, I seek to understand how to represent and visualize influencer communities; how to analyze the strategies that influencers pursue within these communities; and how to understand the effects these influencer communities have on political discourse in the United States. I conclude this dissertation with a collection of reflections, most of which concern how characteristics of influencer communities, such as their embeddedness in power relations and their relationship to the "mainstream," structure which strategies influencers within them take. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]