It might be 'consumer-grade' but man is it well designed. I consider it one of my aspirations to write software as robust as Whatsapp. Sometimes I sit in my cube looking at my utter mess of an Angular app that I develop for a few thousand concurrent users and think, man how the hell am I ever going to get anywhere as close to as how good something like Whatsapp is.

The reason its so good is how it handles edge cases. it brilliantly handles edge cases like when connection is extremely spotty. In 2015 when I was backpacking around Europe I learned this the hard way when iMessage repeatedly hit issues where it would get stuck transmitting mid message and would require drastic things like rebooting the iPhone (because that damn status bar gets stuck and then you have no recourse) whereas no matter how slow/spotty the connection (like 64kb/s or slower), Whatsapp would patiently work and reliably get the transmission through eventually.

In hindsight, I am thankful Zuckerberg had the sense to acquire them. He has so far done a lot less to destroy it than I expected. Can you imagine if they got acquired by some garbage dump of a company like Oracle, or Salesforce, or (shudders) Google? Just think of all the ways they would have ruined it.