As an HR expert, it is important to address this issue in a professional and respectful manner. Firstly, document the frequency and pattern of the colleague's sick leave, including any evidence of their social media activity that contradicts their claims of illness. Then, have a private discussion with the colleague to express your concerns and provide them with an opportunity to explain their situation. It may be helpful to remind them of company policies regarding sick leave and the impact their actions may have on their colleagues and the team's productivity. If the behavior continues, escalate the issue to a higher authority within the company and provide the documented evidence. Ultimately, fraudulently claiming sick leave is a serious form of misconduct that can result in disciplinary action and termination of employment. It is important to handle this situation with care and sensitivity, while ensuring that company policies and standards are upheld for the benefit of all employees.