wiill first i think it,s done by both shape and bones,
i made a short review of a face character that i did, using shape keys and bons ,
you can add as much as you like of sheps to one bone, and it depends on how wil it control it,
ROT LOC or even SCALE ,
check this one out : https://www.youtube.com/watch?v=n3QpcQ2_bY0
also the tread : Face project
i didnt upload the full videos yet but i am in the way to.
sadly i dont have a good internet to make tuts ,but if you have any question just ask,
also i uploaded the model in a blend file if you would like to check it out
run this name out on youtube (Creating Facial Shapes in Blender) by CGMASTER