1. A method for selecting strangers for information spreading on a social network, the method comprising: training statistical models with history data of the information spreading of strangers on the social network, the strangers on the social network being not known to a requester who requests the strangers on the social network to conduct the information spreading; computing information spreading probabilities based on features of the strangers on the social network; computing information reach of the strangers on the social network; computing information spreading probabilities based on a wait time of the strangers on the social network, the wait time being a period of time for the strangers on the social network to initiate information spreading in response to being requested by the requester; computing fitness scores of the strangers on the social network, the fitness scores being a function of: the information spreading probabilities based on features, the information reach, and the information spreading probabilities based on the wait time; ranking the strangers on the social network in a sorted set, based on the fitness scores; selecting one or more of the strangers for the information spreading from the sorted set; and wherein the one or more of the strangers for the information spreading are selected by determining an interval in the sorted set, the interval satisfies an optimization objective of minimizing time for the information spreading within a confidence probability.