Firstly (and many other people have mentioned this already) - you absolutely nailed the hair! It is definitely the most striking feature of this sketch. (And most certainly is not an easy thing to achieve). The contrast you use is excellent, and makes the image really pop out in a more realistic manner.
The mouth is skilfully done as well.
The composition works well, and I like that it's all about the hair, but still has substance to the piece with the face peeping out. It's very striking.
The only critique that I have about this sketch is the nose. The angle of the shading makes it appear slightly too high up - or as if the bridge of the nose is too short. Perhaps it would be better to define the ball of the nose a bit more to bring it more forward (it comes across as a little flat) - this will help to push it out and down, and appear longer.
But overall, this is truly an excellent piece, done with obvious talent and skill. Keep up the great work!