If you have a closed system, a platform with users inside that login with name and password, it's not hard to introduce "social network" features into it. It was always the plan at ZEBEDEE to introduce such a thing, but much better than a closed social network just for ZBD users is if one such a thing can plug into the outer world of Nostr. Therefore ZBD Social is both an internal social network and a network that is open to the external world through Nostr.
The ZBD app already includes a custodial Bitcoin Lightning wallet and the target userbase doesn't want to care about keys and prefers email and password as the login mechanism to a trusted platform, therefore the ZBD Social is a custodial Nostr client. ZBD users also may be running their app on low-spec phones and low bandwidth, and since the key is already custodial it makes more sense to have all the Nostr logic for each ZBD user to be done on a ZEBEDEE server, instead of in the device itself, therefore the Social section on the ZBD app is just a thin client to an internal API.
Doing the correct thing given the constraints
In order for Nostr to scale, people must be able to host their notes in whatever relay they want and their followers must still be able to find these.
With that goal in mind, the ZBD Social server keeps track of all associations it can find -- in event hints, kind 3 and kind 10002 events, nprofile and nevent codes and the bare fact that a given event from someone was found in a given relay -- and uses that information to estimate the best possible set of relays to be used to fetch notes for each Nostr user, along with some variance to account for the fact that these sets are dynamic.
Whenever a ZBD user wants to read notes from any external Nostr user -- either because they've opened on that user's profile or because they follow that user and are browsing their classic "home feed" with notes from everybody they follow -- the ZBD Social backend will gather the best relays for that given user and open new subscriptions -- if there isn't already a subscription open -- for that user. If there are already other subscriptions open for other users in that same relay, the subscriptions will be merged in order to not spam external relays.
As they come in, notes from external users are cached in a way that they are automatically evicted as soon as memory is low and they haven't been accessed for a while. Browsing through old notes is done through paging these cached notes, indexed by author.
The wss://nostr.zbd.gg relay
The ZBD relay stores all events emitted by ZBD users. It runs strfry with a plugin that makes it interact with the rest of the backend. It is replicated accross multiple instances using strfry's native syncing capabilities and serves both as a normal relay interface to which external Nostr clients can talk normally and as a database that can be queried by the internal backend (turns out strfry is not only a Nostr relay, it is also a mechanism to turn LMDB into a cloud-native datastore).
This makes it easy to have a dedicated tab on the app with the feed of all the other ZBD users, which is effectively the same as browsing just wss://nostr.zbd.gg from any other Nostr client -- see, for example, Coracle, nostrrr, nostr.com or using the CLI: nak req -l 10 --stream wss://nostr.zbd.gg | jq.
It also contributes to the future world of Nostr in which niche relays can be browsed individually to enhance the experience of normal social interactions. For any given note, for example, you should be able to see "what are the ZBD users commenting about this" or "what are the gold enthusiasts saying" and so on.
Ideas for the future
Being a Nostr custodian in a platform that offers Lightning payment services and other third-party integrations for its existing userbase, it's easy to see how ZEBEDEE can start bridging Nostr into more things inside its domain.
For example, in the future ZEBEDEE could offer a way for game vendors to plug in a social networking layer into their games and that wouldn't be just an API to a proprietary platform, but a bridge to the real Nostr world that integrates seamlessly with the ZBD app for ZBD users, but works in Nostr-native mode for any Nostr user. Another use case could be powering social features for music and entertainment apps. Another very obvious use case is a NIP-58 badges system that games and other "gamified" services and apps can use.
In a not distant future, I imagine we'll see also integrations with the ZBD browser extension and NIP-07, Nostr features with the Telegram and Discord bots, and NIP-53 integration with ZBD Streamer (but I am not officially announcing anything).