In online social network (OSN), understanding the factors bound to the role and strength of interaction(tie) are essential to model a wide variety of network-based applications. The recognition of these interactions can enhance the accuracy of link prediction, improve in ranking of dominants, reliability of recommendation and enhance targeted marketing as decision support system... In recent years, research interest on tie strength measures in OSN and its applications to diverse areas have increased, therefore it needs a comprehensive review covering tie strength estimation systematically. The objective of this paper is to provide an in-depth review, analyze and explore the tie strength in online social networks. A methodical category for tie strength estimation techniques are discussed and analyzed in a wide variety of network types. Representative applications of tie strength estimation are also addressed. Finally, a set of future challenges of the tie strength in online social networks is discussed. (read more)