From Kotaku:

How thoughtful! A PlayStation Network promotion wrapped in a thank you note sent to selected folks. Sony, you should’ve have. No, really. You shouldn’t have.

A better idea would have been to include at least a promotional coupon or exclusive demo available to the users you’re emailing. They’ve already proven a willingness to spend on the network, so why risk alienating some of them with a disingenuous message like the one Sony sent