Since the Hyperdrive FS is read/writable from the browser, you can use web apis and in-browser tooling to work on the files. This even makes it possible to write applications by publishing posts/comments/votes/etc as files in the user's personal hyperdrives.
By default, every user creates a personal drive to act as their identity. We use that to act as a kind of social network, where the site is both their identity and their public data locker. The "twitter clone" is called Blahbity-blog, and it's very simple: it just reads the files you write to /microblog and merges that into a feed ordered by ctime. Typically you publish .md files but it can be anything, including .html (which runs in an iframe).
The p2p network uses a DHT (https://en.wikipedia.org/wiki/DHT) with builtin hole-punching (https://en.wikipedia.org/wiki/Hole_punching_(networking)) to connect people's devices. You can also send messages between peers using that network with an API called PeerSockets.