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; and selecting one or more of the strangers for the information spreading from the sorted set.