Ask HN: Why can WhatsApp handle so much load when Twitter struggled?

For years, Twitter struggled with the amount of traffic they had (e.g. Fail Whale) when WhatsApps hasn't been and they have a lot more traffic.

Why, especially given that they both have similar problems sets (sending messages).

To give some stats, WhatsApps is processing 50B msg / DAY and sustaining 580k msg/sec[1].

Twitter is at best is 143k msg/sec, and that's note even sustained [2].

Furthermore, WhatsApp is achieving 2M tcp connections on a single node and 200k msg/sec. [4]

Can anyone explain why WhatsApp has been able to be so performant in comparison to Twitter?

[1] https://twitter.com/WhatsApp/status/286591302185938946

[2] https://blog.twitter.com/2013/new-tweets-per-second-record-and-how

[3] http://blog.whatsapp.com/index.php/2012/01/1-million-is-so-2011/

[4]slide 16 http://www.erlang-factory.com/upload/presentations/558/efsf2012-whatsapp-scaling.pdf