Earlier quoted context omitted.
You're so close to what I am currently building that I feel like I have to chime in. I left Apple's security team a few months ago to go solo and build an app that is similar to Facebook/Instagram and Snapchat in terms of functionality and UX but uses the Olm protocol (similar to Signal) to protect all content. So, fear not - something is being done, and I'm sure I'm not the only one working in this problem space. I…
How are you going to recreate the main selling point of facebook, i.e. the Newsfeed ? If things are encrypted, how are you going to determine relevance, etc.
When you post, the content is sent as an encrypted message that can only be decrypted by people who should be able to see it (either all of your friends or just a subset of them). The client automatically takes care of key management and is responsible for keeping track of sent & received posts, comments, etc; it renders a news-feed-like UI on top of that data. The end result is a familiar UX on the surface, but the underlying mechanism for transmitting data between users is far more secure than the traditional approach of using a monolithic database that contains everyone's data in plaintext.
> If things are encrypted, how are you going to determine relevance, etc.
The client is solely responsible for that. For now the "news feed" is strictly chronological, but I plan to augment it later by prioritizing posts that might be particularly interesting to the user. There are plenty of ways to make those decisions locally.