Protection of On-line Social Networks (OSNs) resources has become a primary need since today OSNs are the hugest repository of personal information on the Web. This has resulted in the definition of some access control models tailored to the protection of OSN resources. One of the key parameter on which access control decisions in OSNs should be based is represented by the trust between OSN users. A well-known approach for the management of trust relationships is represented by trust negotiations. In this paper, we show how access control and trust negotiation can be combined in a framework for the protection of OSN resources. Moreover, we show how the outcome of a trust negotiation can be exploited to dynamically adjust the trust level between OSN users.