- They have a reasonably similar number of users (a fraction, but a large one).
- Mistakes cost a lot, so they have to be a lot more careful. It's a lot easier to make money hacking a bank than hacking facebook.
- They have to comply with all sorts of regulations.
- They probably don't trust their own employees to not be trying to commit fraud.
- They have to parse data on a scale that is likely similar or greater than facebook's. To detect fraud/lost credit cards/.... To decide who to give loans to. To price insurance. To decide how to trade stocks. ...
- They have to run a physical fleet of devices in the field, outside of their control, that have to give people the right amount of money ~100% of the time.
At a glance I see that Facebook has something like 300 petabytes of data [0]. I've worked at a bank, my team had something more like 10, but I don't think much of it was things like video that are just naturally huge. BOA is also approximately an order of magnitude bigger than the bank I was at.
One rumor I heard while there was that there had been a bug in one of our mobile apps that had been costing us a million dollars a day in server time.