Try with this:
var width = 640, height = 480;
var widthHalf = width / 2, heightHalf = height / 2;
var vector = new THREE.Vector3();
var projector = new THREE.Projector();
projector.projectVector( vector.setFromMatrixPosition( object.matrixWorld ), camera );
vector.x = ( vector.x * widthHalf ) + widthHalf;
vector.y = - ( vector.y * heightHalf ) + heightHalf;