[jude-]

Blockstack engineer here.

The protocols under the hood ensure that each user chooses where their data gets stored -- all that's required is that wherever it is, it's available via HTTP(s) so it can be loaded into your Web browser. As such, Blockstack-powered applications hold no user state -- the user takes their data with them, and "brings their own storage" to the application. The blockchain component acts as a discovery mechanism, whereby any user can discovery any other user's public keys and storage locations, given a username.

Blockstack PBC gives users a small amount of free storage space to help onboard them, and runs some publicly-routable Blockstack nodes. However, nothing stops you from electing to store your data in your own storage provider (e.g. S3, Dropbox, your personal server, etc.).