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Followers are established by connecting to bittorrent swarms. From the paper: "The last network is a collection of possibly disjoint “swarms” of followers, based on the Bittorrent protocol, which can be used for efficient near-instant notification delivery to many users."

Furthermore, not all clients need to store and reseed all messages they receive. More seeders is obviously better for network health, but it's not always a reasonable option. The paper suggests that clients can choose to be "achivists" which means that they keep messages and seed them to others (so it's optional). Clients like mobile phones could easily disable this behavior.