Structure, Topics, and Diffusion Effects of Bluesky Starter Packs
User discovery is a central challenge in online social platforms, particularly during onboarding. Bluesky, a decentralized microblogging platform built on the AT Protocol, introduced starter packs: curated collections of accounts that users can follow in a single action to bootstrap their social network. In this paper, we present a large-scale empirical analysis of more than 50,000 English-language starter packs and over 600,000 associated users. We characterize their structural organization, topical composition, and impact on content diffusion. Our results show that starter packs form a highly interconnected ecosystem with substantial overlap across packs that largely reflects pre-existing communities. Topic modeling reveals a skewed landscape dominated by automatically generated personal packs alongside several thematic communities, which exhibit similar structural properties but markedly different adoption patterns. Finally, a matched event-study analysis shows that inclusion in a starter pack is strongly associated with a substantial increase in short-term repost activity.