Any companies on that list using Gmail as their primary email service are foolish. But I'm not sure why you think I was somehow equating Facebook with Google simply because I don't agree that everyone has Gmail and is using it.
I use google apps to host my email for my company, with all emails backed up to our server. Why is that foolish?
Ah, now here's a different thing: should a comet fall on Google's data centres, you still keep your data. I infer that you probably thought about privacy risks. I also suppose your company's email address isn't "@gmail.com". That would be foolish.