In the evolving realm of online social networks (OSNs), assessing cybersecurity risks and implementing effective countermeasures are crucial for practitioners. This chapter confronts this challenge, beginning with an extensive literature review that explores the range of threats, vulnerabilities, and attacks prevalent in OSNs. It differentiates between general cybersecurity risks and those unique to OSNs, with a special focus on user-side vulnerabilities. The chapter critically analyzes risk assessment and security assessment, highlighting their distinct yet interconnected roles in cybersecurity. Various risk assessment methods are evaluated for their applicability to OSNs, alongside a discussion of both technical and non-technical countermeasures for risk mitigation. Concluding with key findings and future research directions, this chapter offers a comprehensive guide for understanding and tackling the complex cybersecurity challenges in online social networks.