1. A face annotation method adapted for a current owner to annotate contacts in online social networks (OSNs), the face annotation method comprising: providing a pyramid database access control (PDAC) module which consists of a plurality of pyramid database units and performs a first batch of access control procedure and a non-first batch of access control procedure, wherein the pyramid database unit includes a plurality layers of database and is constructed according to social relationship information; providing a multiple-kernel learning face recognition (MKL-FR) module of the current owner implemented through the use of a MKL classifier unit which uses a MKL algorithm to achieve a face identification, wherein the MKL classifier unit is trained with the pyramid database unit accessed in OSNs and includes an offline learning procedure and an online recognition procedure; and if the MKL-FR model is not able to identify query faces, providing a multiple-kernel learning face recognition fusion (FRF) module to perform a collaborative face recognition strategy by utilizing a user who features a highest priority rule within a collaborative face recognition framework based on characteristics of OSNs.