A bit of Friendica background:
Friendica's "Facebook-type" capabilities are its core features – things like sharing posts, photos, and connecting with friends. These features exist independently of any integration with other platforms (read Activitypub, Diasporia, and Bluesky). This is what this URL is for. Letting my server have a Facebook-type relationship with the other, and only the other Friendica servers. Mastodon is not invited to this party. So, if anyone wants to share their Facebook-type timelines and calendars with my server, I need to get into dir.friendica.social. If anyone on my server hopes to have a Facebook-like relationship with someone on another Friendica server, then that server has to be in dir.Friendica.social.
Or, if people wish to have their own isolated community of servers, anyone can set up a private equivalent of dir.friendica.social. The directory code is open source, and apparently people have indeed set up closed Facebook-like communities. Then the URL might point to a private community of Friendica servers that specialize in, perhaps, archeology enthusiasts, that won't include other Friendica servers.
Friendica's ActivityPub integration is an additional and independent layer that allows it to communicate with other platforms in the Fediverse, like Mastodon. It has nothing to do with it's Facebook-type feature.
So, you could use Friendica just for its "Facebook-type" features without ever interacting with the Fediverse. Or, you could use its ActivityPub integration to mainly interact with people on Mastodon. Or use both. I am currently only using the Activitypub integration and will play later with the Bluesky integration.
Think of it like a house with a phone line. The house itself is functional on its own (Friendica Facebook-like server), but the phone line adds the ability to connect with the outside world (via Activitypub, Diasporia, and Bluesky).
The URL tin question is a pointer to dir.friendica.social to get all the other Friendica servers for playing in the Facebook-like feature. It isn't for getting me into the directory.
