Problem in database design

I think there are some problems in the chapter. In requirement we said we will take that only user will post feed and in db design we are taking entity post also.

In feedItem table , what will be userId and entityId, when

  1. the user is making a post/
  2. also how would you identify the type of entity from entity id. there are lot if missing dots.

Course: Grokking the System Design Interview - Learn Interactively
Lesson: Designing a URL Shortening service like TinyURL - Grokking the System Design Interview