Restrictions arising from the COVID-19 pandemic have limited opportunities for older people to participate in face-to-face organised social activities. Many organisations moved these activities online, but little is known about older adults' experiences of participating in those activities. This paper reports an investigation of older adults' experiences of participating in social activities that they used to attend in-person, but which were moved online because of strict lockdown restrictions. We conducted in-depth interviews with 40 older adults living independently (alone or with others). Findings from a reflexive thematic analysis show that online social activities were important during the pandemic for not only staying connected to other people but also helping older adults stay engaged in meaningful activities, including arts, sports, cultural, and civic events. Online activities provided older adults with opportunities to connect with like-minded people; share care, encouragement, and support; participate in civic agendas; learn knowledge and develop new skills; and experience entertainment, distraction, and mental stimulation. Our participants had diverse perceptions of the transition from in-person to online social activities. Based on the findings, we present a taxonomy of multi-layered meaningful activities for older adults' digital social participation and highlight implications for future technology design.