Discovery is crucial.
A social network can be thought of as a kind of search engine: you're trying to find people, posts and resources that are relevant to your interests.
On the other hand, a social communication app works by allowing you to connect to your existing friends. So for example, with an app like Peach, finding new people or searching for posts matters less.
The really hard question for the first case is: how do you combine searchability with end-to-end encryption?