How to fan-out/distribute 500,000 records with Firebase?

Question

Requirements:

• Users can register to company & unsubscribe freely.
• Users should get only the posts of companies they are registered to.
• Users shouldn't see posts they already liked/disliked

Currently, what we are doing is, when a user joins a company, we copy the relevant posts to the user User_Feed, and the client fetches the posts.

Now assume we have 40K posts & 500K users. Each user that joins a company, we need to copy 40K of data to the user feed. Or when a company posts a new post, we need to build a huge fan-out object to pass to 500K USER_FEEDs.

This is not scalable.

David East in his post states that the fan-out technique supports millions of records, but even in his example, how can he handle 1M followers?

We feel like we're going about it the wrong way.
Is there a better solution to our problem?

We currently have the following structure:

Companies(~20 companies) 
{
    companyId:  {
        ...comanyInfo
    }
}

Users (~500K users)

  uid: {
     ...userInfo
   }

User_Companies (500k * max(20 companies)) 

 uid: {
    company1: true,
    company2: true,
    ....
 }  


Company_Users (20 * max(500K users)) 

     companyId: {
        uid1: true,
        uid2: true,
        ....
     }   

 Company_POSTS (2K posts/company) 

 {
    companyId: {
        postId: {
            ...postInfo
        }
    }
 }

 User_Feed (For each user (500K))
  {
    uid: 
        posId : {
            like: true
        }
 }

This post is also opened in Firebase Google Group