Very few insurgent groups take and occupy territory in the way that conventional military forces do. When discussing the influence of insurgents, it's more worthwhile to talk about the territory through with they can travel and operate unimpeded. By that metric, IS does "hold" almost half of Iraq, in that they can travel throughout most of Northern Iraq without fearing attack from the Iraqi or Syrian militaries [1].

[1] http://static3.businessinsider.com/image/5446d4ca6bb3f77b1e8...