I'm fairly sure that when they first introduced support via Twitter, Facebook etc, it was aimed at those who wanted to send a message but didn't need an instant response ie they could be sent a reply and read it whenever they wanted to. I don't think that it was marketed as instant messaging.
I agree with you about the phone hours though @
AndrewR, what these businesses seem to fail to understand is that if you get the basics right in the first place, you shouldn't need as much support. The systems are becoming more complex and therefore there's more inherent problems, and the customer support that they do have don't have the skills/training/knowledge to give a good level of service. It's not just e.onnext, it's widespread.
This is why I always try to phone businesses as much as possible, if you send any kind of message it rarely gets read properly so you have to do it again, and again....