Do USB sockets draw power when not in use?
It is very unlikely that they won't.  Nearly all AC to DC converters, by design, draw a very small idle current when you aren't charging any devices.  Cheaper designs consume more while some good designs consume less, but unless there's a physical switch on the USB socket it will always consume some power while waiting for a device to be plugged in.

These are new sockets to the market, and unless you have a compelling reason to avoid a plug in USB charger I would recommend holding off anyway.  Not only do you have the continuous current draw (which, honestly, is going to be less then $5.00 per year even if the device is terribly inefficient) but there's the likelihood of the socket failing, and it's not nearly as easily or cheaply replaced as a plug in charger.  Further it may not charge all devices at their full current.  While the USB association has suggested designs for high power charging (above 500mA), many manufacturers (notably, Apple) still use their own scheme for detecting 1A and 2A charging currents.

Most phones and PDAs will only charge at 1A or greater when connected to their own charger, and at the slower 500mA rate when they can't detect their own charger.

There are probably some applications and situations where it might be useful, for those prone to losing their chargers, for instance, but there are many reasons to reconsider using them.