I'd also suggest that it's not just scale; the kind of reliability Facebook needs is fundamentally different than what a bank needs. Broadly speaking, Facebook needs the site to keep working as well as possible even if some subservice fails, and a bank needs a subservice not to fail. I'm summarizing here and I know it; clearly neither of them is actually on the absolute extreme end, as Facebook needs authentication to work and a bank may not care if the interest rate display widget on their customer banking app fails to load a couple of times. But I'd still suggest there's enough difference between the requirements to be a fundamentally different domain.
Even in "the cloud" things differ between services. A social media app has very different reliability requirements than a backup cloud.
Well actually there are many sub services in a bank that can go down without major impacts. The two major banks I use have weekly planned outages of features like old statement retrieval, person to person payments, ACH transfers, etc. Basically everything in the web interface could experience outages without any major crisis.
As long as ATM requests always work, nobody really seems to care.
"Broadly speaking, Facebook needs the site to keep working as well as possible even if some subservice fails, and a bank needs a subservice not to fail."
One of the reasons many of them stick with mainframes, AS/400's, and NonStop systems for backends. ;)
Even in "the cloud" things differ between services. A social media app has very different reliability requirements than a backup cloud.