The middle icon under everyon's avatar now has changed, it should appear to be 2 little talk bubbles.
This is a new feature, called vbPaging. It's an instant messenger for the site only, featuring instant pop-up boxes. All members now have access to it, so you can use it as a quick alternative to private messaging or whatever else you want.
As with private messages, I will state here clearly and immediately that I DO NOT READ THEM. In fact, the settings are such that as soon as someone closes their little window, the message is no longer on the server in any way.
You may also start getting notices from mods using this format, rather than private messages. It's quicker, easier, and puts a quick end to bad behavior, hopefully.
You may always turn your vbPager options off in your UserCP, if you choose not to receive them. You also won't get them from anyone you've put on your ignore list. However, mods will always be able to contact you via this method.