  We study the nodal set of solutions to equations of the form $$ (-\Delta)^s u
= \lambda_+ (u_+)^{q-1} - \lambda_- (u_-)^{q-1}\quad\text{in $B_1$}, $$ where
$\lambda_+,\lambda_->0, q \in [1,2)$, and $u_+$ and $u_-$ are respectively the
positive and negative part of $u$. This collection of nonlinearities includes
the unstable two-phase membrane problem $q=1$ as well as sublinear equations
for $1<q<2$. We initially prove the validity of the strong unique continuation
property and the finiteness of the vanishing order, in order to implement a
blow-up analysis of the nodal set. As in the local case $s=1$, we prove that
the admissible vanishing orders can not exceed the critical value $k_q= 2s/(2-
q)$. Moreover, we study the regularity of the nodal set and we prove a
stratification result. Ultimately, for those parameters such that $k_q< 1$, we
prove a remarkable difference with the local case: solutions can only vanish
with order $k_q$ and the problem admits one dimensional solutions. Our approach
is based on the validity of either a family of Almgren-type or a 2-parameter
family of Weiss-type monotonicity formulas, according to the vanishing order of
the solution.
