  The spread of behavior in a society has two major features: the synergy of
multiple spreaders and the dominance of hubs. While strong synergy is known to
induce mixed-order transitions (MOTs) at percolation, the effects of hubs on
the phenomena are yet to be clarified. By analytically solving the generalized
epidemic process on random scale-free networks with the power-law degree
distribution $p_k \sim k^{-\alpha}$, we clarify how the dominance of hubs in
social networks affects the conditions for MOTs. Our results show that, for
$\alpha < 4$, an abundance of hubs drive MOTs, even if a synergistic spreading
event requires an arbitrarily large number of adjacent spreaders. In
particular, for $2 < \alpha < 3$, we find that a global cascade is possible
even when only synergistic spreading events are allowed. These transition
properties are substantially different from those of cooperative contagions,
which are another class of synergistic cascading processes exhibiting MOTs.
