System requirements
Functional:
users should be able to publish tweets
users should be able to follow other users
users should get a personalized feed of tweets published by people they follow
users should be able to like and comment on tweets
Non-Functional:
system should be scalable to many users
system should have high availability for better user experience
users should be able to quickly see their personalized feed
we want both reads and writes to be fast, but reads especially since likely there will be more reads than writes
Capacity estimation
1 billion users
each user publishes 10 tweets a day
each tweet is 100 characters long on average so 100 bytes per tweet
so 1TB of data per day over say 400 days = 400TB per year
API design
Define what APIs are expected from the system...
Database design
Defining the system data model early on will clarify how data will flow among different components of the system. Also you could draw an ER diagram using the diagramming tool to enhance your design...
High-level design
You should identify enough components that are needed to solve the actual problem from end to end. Also remember to draw a block diagram using the diagramming tool to augment your design...
Request flows
Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...
Detailed component design
Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...
Trade offs/Tech choices
Explain any trade offs you have made and why you made certain tech choices...
Failure scenarios/bottlenecks
Try to discuss as many failure scenarios/bottlenecks as possible.
Future improvements
What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?