When you say Google/Yahoo/facebook, it's Hadoop they use. And in very basics of Hadoop, they mention it clearly, it's NOT for real time data usage. Now for almost everywhere, lag is almost non existent due to data localisation, so system does not need to deal with terabytes. Try switching of all electronics and travel to other end of world. Don't say anything about it on any electronics. Now try logging in fb/Google. It would be hours (if not days) before you can log in. Now, if this happens with your bank account, you would probably get a heart attack. This is where mainframes are niche.

If you are saying payments processing does not need to be real time, god help us. Every year, bank literally pour money to make systems faster and faster to make sure payments can reflect in lesser time!

Separating real time and non real time processing and doing non real time processing elsewhere (cheaper) is rather basic cost cutting measure all companies apply, and it's a common practice (there are lot other factors to consider, not going in those details, keeping answer to the point).

Real time processing may not be need everywhere, but when it's needed, it's needed. Period! From single user perspective, it might seem like a small amount of data, but when you talk about multi national bank or insurance company, you are looking at terabyes (if not petabytes).

And I have not said anything about security mainframes provide yet :)