The growing trend to social networking and increased preva- lence of new mobile devices lead to the emergence of mobile social networking applications where users are able to share experience in an impromptu way as they move. However, this is at risk for mobile users since they may not have any knowledge about the users they socially connect with. Trust management then appears as a promising decision support for mobile users in establishing social links. However, while the literature is rich of trust models, most approaches lack appropriate trust bootstrapping, i.e., the initialization of trust values. This paper addresses this challenge by introducing proximity-based trust initialization based on the users' behavioral data available from their mobile devices or other types of social interactions. The proposed approach is further assessed in the context of mobile social networking using users behavioral data collected by the MIT reality mining project. Results show that the inferred trust values correlate with the self-report survey of users relationships.