  There is a long-standing belief that in social networks with simultaneous
friendly/hostile interactions (signed networks) there is a general tendency to
a global balance. Balance represents a state of the network with lack of
contentious situations. Here we introduce a method to quantify the degree of
balance of any signed (social) network. It accounts for the contribution of all
signed cycles in the network and gives, in agreement with empirical evidences,
more weight to the shorter than to the longer cycles. We found that, contrary
to what is believed, many signed social networks -- in particular very large
directed online social networks -- are in general very poorly balanced. We also
show that unbalanced states can be changed by tuning the weights of the social
interactions among the agents in the network.
