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

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.