From my understanding, an essential part of Solid are the specifications and standards it proposes. I think of Solid pods as a standardized Dropbox. So without knowing the specific implementation of the pod, developers still have a common interface to work and store data on the pod.
The advantage over a “classic” server, is this common interface: You could use any app with your pod, which makes use of these specifications. This should make it easier to select from a multitude of apps and switch them if necessary. If it would be a normal server, the app would have to be customized to the behaviour of it.
Linked Data will be useful to make the apps interoperable and to reuse data which other apps generated (but I’m no expert in this…)
Yes, there’s been a discussion about this already in the forum somewhere…¹ I think one idea to solve this was a decentralized webId, but I’m not sure how this would work.
I also want to note, that Solid also enables several applications which don’t require a backend. The user loads the application, points it the webId, and then the app uses the preferred pod of the user as a data storage. For example, the ToDo app itself won’t need a backend. It would only send requests to the users pod.
So in the Facebook example, it possibly wouldn’t have any servers itself. And when changing the webId one wouldn’t need to notify “Facebook”, but rather tell the friends that the webId changed so they still know where to find me. And then use a different webId in the app. I personally have some doubt that it will be possible to create a Solid-Facebook with the same functionally without a backend, but I’m looking forward to the different social apps which will emerge. Also questionable if one even would like to copy FBs functionality… :))