A blog network, composed of blogs and their relations, may exhibit two different propensities characterized by the purpose of use: an information-oriented propensity and a friendship-oriented propensity. Both propensities coexist in a blog network, and the degree of these propensities may play an important role in business and policy decisions of blog-related business. In this paper, we propose an automated method for determining the propensity values of a blog network. First, classification is used to judge the propensity values of the relation between two blogs. Then, by adding up the propensity values of all the relations in the network, one determines the propensity values of the whole network. Through extensive experiments using a large volume of real-world blog data, we demonstrate our method achieves a high level of accuracy in determining the propensity values of a relation. The results also suggest the applicability of our approach for determining the propensity values of a network.