Twitter has ads serving infra, recommendation systems (timeline, notifications, events, users), user generated events, prediction systems (ads), user graphs. The complexity is from processing and persisting exabytes of data in company owned datacenters. eg. Twitter stores images, videos, user events, user data, tweets/replies. WhatsApp has little persistence outside of metadata maybe? But your messages are not stored in a FB datacenter and if they are I'd be concerned. You can read about their infra in their blog. Comparing p2p messaging versus a distributed social media site with mountains of data and years of iteration in ML systems does not make sense.

https://blog.twitter.com/engineering/en_us/topics/infrastruc...