Online Social Networks (OSNs) are more and more popular recently, people may through them interact with each other for the purpose of social intercourse. The client/server OSN architecture brings about the bottleneck of bandwidth and computation. It leads to the scalability problem and the communication latency increases as users grows. This paper proposes a bandwidth- and latency-aware peer-to-peer (P2P) instant friend cast scheme for every user (or peer) in OSNs to construct a friend cast tree (FCT) to send instant messages to all of its friends. A lightweight server is responsible for only easy tasks, such as logining and maintaining peer information, to facilitate the tree construction. A peer logins to the server to obtain the list of friends and their Vivaldi coordinates, which are computed by every peer in a distributed way to estimate the latency between peers. The proposed scheme also uses Available Out-Degree Estimation (AODE) to evaluate the proper out-degree of a peer, and then uses Degree-Adapted Greedy Tree Algorithm (DATGA) to construct FCT. The scheme is simulated and compared with other relevant ones to show its advantages.