  We provide new direct methods to establish symmetrization results in the form
of mass concentration (i.e., integral) comparison for fractional elliptic
equations of the type $(-\Delta)^{s}u=f$ $(0<s<1)$ in a bounded domain
$\Omega$, equipped with homogeneous boundary conditions. The classical
pointwise Talenti rearrangement inequality is recovered in the limit
$s\rightarrow1$. Finally, explicit counterexamples constructed for all
$s\in(0,1)$ highlight that the same pointwise estimate cannot hold in a
nonlocal setting, thus showing the optimality of our results.
