Facebook asks System Design questions to test your design skills and your ability to work with complex and scalable services. Your performance in these interviews determines what position and salary you will be offered. If you have commendable design skills, you will get a better offer. A good resource to prepare for this interview: Grokking the System Design Interview.

Recently I’ve conducted many system design interviews (SDI) and like coding interviews candidates who haven’t spent enough time preparing for SDIs mostly perform badly. This gets aggravated when you are interviewing at the top companies like Google, Facebook or Amazon. In these companies, if a candidate doesn’t perform above average, they have a limited chance to get an offer. Also, a good performance in such interviews always results in a better offer (financially), since your performance reflects upon your ability to work with complex systems.

Also, all such big companies like candidates who are familiar with distributed systems concepts like Consistent Hashing, Data Partitioning, Load Balancing, Redundancy and Replication, CAP Theorem, etc.

A few sample design problems are:

• Designing a URL Shortening service like TinyURL (Design Gurus)
• Designing Instagram (Design Gurus)
• Designing Facebook’s Newsfeed (Design Gurus)
• Designing Twitter (Design Gurus)
• Designing Facebook Messenger (Design Gurus)
• Designing Youtube or Netflix (Design Gurus)
• Designing Web Crawler (Design Gurus)
• Designing Typeahead Suggestion (Design Gurus)
• Designing Uber (Design Gurus)
• Designing Ticketmaster (Design Gurus)

Hope this helped.