Also, graph algorithm’s likely don’t scale linearly, 10 times more edges may have much much more resource usage.
So you're saying WhatsApp had no critical other teams, but Facebook bought them anyway for a record breaking sum of money at the time, because they looked nice on picture?
When Elon tweets, I'm guessing Twitter doesn't go do 118M write transactions to all his followers. But what it would have to do is flag all 118M accounts as "something as changed". Then the next time one of his followers is on Twitter, which could be a month from now, that flag means "uh oh, better go see what changed", and to do that, all of the follower's "following" accounts have to be accessed to find the change, if there is only 1 change flag per Twitter account.
If Twitter keeps a change flag per "Following" user, that makes finding changes easier, but also means lots more flags to keep updated.
Or, you could go "stateless" and not use flags, but then it seems like you'd be doing a lot of work for users who may not see it for days or months.
Way different problem than 1-on-1 messaging.
if my mom sends a message to the family group it goes to 50 people.
How many billion WhatsApp groups are there?
with the same constraints and problems Twitter has to face (offline users, notifications, keep users messages until they are online again, etc.)
Of course not the same, but the question is, how much harder?
10 times harder?
100 times harder?
Given 1/5 of the users and probably less than 1/200 of the traffic (*) it should be feasible with less than 7,500 employees.
(*) 100 billion messages sent via WhatsApp every day VS 500 million tweets per day