This seems like a very misleading title, the Bluesky PDS is the meant-for-selfhosting thing they distribute, not the bluesky service as experienced and used by most of its users.
Bluesky has a main PDS instance at https://bsky.social that serves almost all of the Bluesky user base.
There is a good overview of the architecture here:
https://blueskyweb.xyz/blog/5-5-2023-federation-architecture
Here’s a snippet from the protocol roadmap they published 3-4 weeks ago [1]:
Multiple PDS instances
The Bluesky PDS (bsky.social) is currently a monolithic PostgreSQL database with over a million hosted repositories. We will be splitting accounts across multiple instances, using the protocol itself to help with scaling.
The main motivation in moving from a big central Postgres cluster to single tenant SQLite databases is to make hosting users much more efficient, inexpensive, and operationally simpler.
But it's also part of the plan to run regional PDS hosts near users, increasing performance by decreasing end-to-end latency.
The most experimental part of this setup is using Litestream to replicate these many SQLite databases (there are almost 2 million user repositories) to cloud storage. But we're not relying on this alone, we're also going to maintain standard SQLite ".backup" snapshots as well.