How much will it cost for Facebook to add one additional user?

CAC= (Marketing+Sales Cost)/# of new customers acquired

However, FB is a network-focused app that runs on friends or connected users. There is a high chance that from a group of 10 people 6 use FB, then there is a high chance that the other 4 will eventually start using FB dude to their friends.

There is no data to know the marketing and sales expense of FB to acquire new users

Let's assume,

Sales and marketing guys expenses: $10,000

PR expenses: $5,000

Ads on other platforms: $20,000

Variable cost: $5,000 (New trend)

Thus total cost: $40,000

If new customers- 

Using Paid Marketing: 1000

CAC: $40/customer

There is no way to understand if the acquired users were acquired organically due to advocacy or paid marketing.

Explanation:

Paid Marketing Flow:

Impressions ->CTR->Landing Page->Creating Account

10,000->8,000->5000->1000

For organic users, it would be 

Landing Page->Creating Account

2000->500

The best thing that can be done is to have different user journeys for each medium to understand if the new users are coming to the website organically or using paid marketing.