This paper seeks to examine the external managerial social networks, and to explain how the involvement in these networks influence on strategic flexibility. Furthermore, it studies the role of perceived uncertainty on this relation. The data was from surveys completed by the managers of 203 Spanish firms. The methodology used was regression analysis. It is observed that external social networks affect strategic flexibility positively through size of the networks. On the other hand, it is found that the environmental uncertainty has paradox moderating effects, negative relation to the size and positive to the strength. Both findings support and extend social capital theory.