Dark mode, an increasingly popular design trend, is a user interface that uses light text and icons on a dark background. As most social media platforms have adopted this trend it’s important that the interface is also designed with usability in mind. Despite seniors' increasing use of social media their needs are often overlooked in the development of digital products compared to younger users. Seniors' needs and challenges often differ from those of younger users, which emphasizes the importance of adapting technology and design to also meet their needs. By understanding how dark mode affects usability for senior users, developers and designers can create user interfaces that meet the needs of a wider audience. This study therefore examines how dark mode may affect the usability for senior social media users, using Facebook as a case. A survey was conducted based on the five quality components of usability: learnability, efficiency, recognizability, error and satisfaction. The result shows that the respondents had a generally negative attitude towards the usability of dark mode on social media regarding all five usability components. We propose a bigger clarity in how to switch to and from dark mode so that seniors easily themselves can choose which mode they prefer to use.