Granted (except Google) - but Twitter, Wikipedia and Craigslist are only big in comparison in terms of traffic. Most banks push more data through their network than most developers can conceive.
If you took all of the votes from the 2000 election in the United States (I'll save you the search - it was just over 100 million), it wouldn't even equal one day's ATM transactions in the U.S., and those transactions are a nonevent.
We all know that there is a lot of data being pushed in banks networks, but your ATM example may not be a very good one. The transactions you are talking about do not occur in a single bank.
Twitter in not only big in terms of traffic. 200 million tweets per day is not small, even if a tweet involves less processing than an ATM transaction.
You can also consider Facebook as an example of website dealing with big data.
If you took all of the votes from the 2000 election in the United States (I'll save you the search - it was just over 100 million), it wouldn't even equal one day's ATM transactions in the U.S., and those transactions are a nonevent.