Many things have already been said. Here´s my 2c:
I don´t think the blur is a result of camera shake. The belt is sharp and so is the left neck, if it would have been camera shake, there should be no sharp areas unless they moved at the same speed the camera shakes

. At close inspection the teeth seem to be somewhat moving and the flash freezes most of the motion. Maybe your model was moving her head/talking? But I don´t think that is the main problem.
Like many other photographers I like to shoot wide open - even for video. There is no rule what you have to do in photography. If you like a particular style like a blurry background it is OK to not have the complete person (or even the complete face) in focus. Anybody who thinks I´m wrong:
take a look at this image and tell me it is bad. It is not shot wide open, but at f4.0 and since the girl is much smaller than your model and the crop is closer, the DOF is even much smaller than in your image. So telling you, you have to stop down to get the right area in focus is not the only option. You may miss more shots shooting wide open, but if you like the style, you will get rewarded.
The focus may be off because you chose the wrong focus point (you say you didn´t, so this is out of the way).
It may also be a focus-recompose issue when you focussed first and then recompose. I do this method all the time, and it works for me but I have read that others have problems with it. It seems to be related to how you tilt your camera.
I repeat myself way too often in this regard, but the lens may have a focus issue (in this image back focus because even the ears are sharper than the eyes). I had that more than once and some cameras have micro adjustment to correct that. This is your last reserve though.
One more thought on the background: if you have people with dark hair, don´t place them in front of a dark background unless you have a lot of light comming from the back, I have brightened the hair a bit to make up for that.
To safe images like this one, you can use the unsharp mask on the face only and blur the rest of the image a tiny bit. Here is my try:
