Your friends provide you with the content, not Facebook. You only need Facebook now because you don’t have a 24/7 agent swapping content on your behalf and presenting it how you like it.
Everyone has email, so you could imagine a social networking app that’s just a thin layer over your email, and every interaction is encoded as an email being sent under the hood. Want to share a picture with your friends? Send an email. Someone wants to comment on it? They just send an email. Etc.
The main purpose of the app would be to offer a nice, device responsive, consistent presentation. Additionally if this were an open, documented standard, an entire ecosystem of “email apps” could emerge.
(Of course as far as your actual email account goes you’d want to auto archive the emails + not get notifications for them, but that’s easily configurable)