The trade-off between the number and closeness of friendships is one of the important features of communication systems. This distinguishes asynchronous text communication through the Internet (lightweight communications) from the face-to-face communication and social grooming of primates (elaborate communications). In this study, we modeled communication as messaging flows driven by edge and node memory mechanisms in order to investigate micro-mechanisms that realize the trade-off law and the differences between the two types of communications. Five patterns of social structures including the trade-off law emerged depending on the strengths of the memory mechanisms. This suggests how communication systems construct different social structures. These results provide insight into the design of online social networks.