In my mind, if technology is a big part of your business then running on other people's computers only has one upside: might be cheaper. Everything else is downsides.

Therefore, A) companies that operate at a scale where the only upside disappears, and B) companies where the cloud saving outweights their expected liability cost, both will have their own datacenters. Facebook is clearly in category A, RenTech is clearly in category B. This is my understanding anyway.

Public cloud is not cheap. Public cloud is really expensive. The value of public cloud is that it's flexible (my current employer, which is massive, uses AWS in part because it has poor governance of leased data centres) and has hosted offerings that save time and operational overhead - it's quicker and easier to use SQS and DynamoDB than to host Kafka and Cassandra. Cheap it is not, however.