The upcoming WebRTC-based browser-to-browser communication services present new challenges for user discovery in peer-to-peer mode. Even more so, if we wish to enable different web communication services to interact. This paper presents Identity Mapping and Discovery Service (IMaDS), a global, scalable, service independent discovery service that enables users of web-based peer-to-peer applications to discover other users whom to communicate with. It also provides reachability and presence information. For that, user identities need to be mapped to any compatible service identity as well as to a globally unique, service-independent identity. This mapping and discovery process is suitable for multiple identifier formats and personal identifying properties, but it supports user-determined privacy options. IMaDS operates across different service domains dynamically, using context information. Users and devices have profiles containing context and other specific information that can be discovered by a search engine. The search results reveal the user's allocated globally unique identifier (GUID), which is then resolved to a list of the user's service domains identities, using a DHT-based directory service. Service-specific directories allow tracking of active endpoints, where users are currently logged on and can be contacted.