TPF is an OS, transaction processing & database system that was written in mainframe assembler and deployed in 1979 to run SABRE and a few other core transaction systems (VISA, AMEX, Marriott hotels, etc.) - maybe 100 customers still use it today - and it was a descendent of a previous assembler system developed in the 1960's.
It probably was (and is) the closest system to "web scale" through the 80's and 90s's in that it ran some of the highest volume workloads globally (e.g. 5000-10000+ tps back in the 80s / 90s, 25000+ tps today) at 99.99% availability. In context, Twitter hits 10-15k tweets per second during big sports events.
These days it's extremely expensive and slow to make changes to TPF relative to modern technology (as it traditionally required S/370 or S/390 assembler, though these days requires C). IBM has a newer version called z/TPF which makes it easier to code in C or C++ and communicate with the outside world via web services, etc.
A very different world...
maybe 100 customers still use it today
Indeed it does seem like there are only a few companies that use this stuff. Like you said, you had to be a pretty big company with massive amounts of transactions to merit it. He's actually contracting with the IRS right working on some newer systems to handle your tax returns. (He tells me every year it's a miracle you get your tax returns every year on the old system)
Whenever he's looking for a new gig he basically has a set list of contacts he goes through. It seems like almost all the developers of these systems know each other.
I remember being at a startup and being impressed that we were doing a million dynamic pages per hour. Then I got the chance to see a real system at a big company pumping out orders of magnitude more traffic than that per second. (I am not at liberty to share exact numbers.) And doing constant processing on that traffic.
I doubt your father would laugh at systems like that if he learned what they manage to handle.
Let's just say that any semi-reasonable back of the envelope will give you some truly impressive numbers. I don't think that I should say any more than that.
Now consider that this data is constantly being processed to figure out things like click through rates, trending requests, and much, much more. And that data is then fed back into the live site.
The implied transaction rate should be enough to impress the average mainframe user.
I can't recall any specific stats or stories right now but you can imagine the discussions between a father who develops primarily with Assembly and a son (me) who ended up being a .NET developer working on various startups. We've had a few talks about the 'cloud' mostly resulting in him laughing at what we call 'web scale'.