Global ageing of the population is deeply affecting the everyday lives of the frailest, by exposing them to increasing isolation and loneliness that, in turn, can cause or accelerate cognitive decline. The use of digital technologies and, in particular, social networks can be an effective tool to mitigate this phenomenon, by helping older age people to stay connected and be stimulated in the cognitive and physical sphere. In this work, we present a platform called Community-Based Activity Center (CBAC) a central tool developed within the scope of the European project Movecare, an effort to leverage intelligent Ambient-Assisted Living technologies to promote active well-being for the elderly. CBAC conveys cross-domain stimulation in the cognitive, physical, and social scopes by providing a virtual community where different types of activities can be carried out alone or together with caregivers, members of the family, friends, and peers. We followed a modular approach, developing a flexible platform that be integrated with AI-based recommendations and that allows for transparent monitoring. The effectiveness of our platform has been extensively tested in a preliminary usability test and in a pilot experimental campaign that involved 25 selected seniors on a time span of 10 weeks.