A system and a method for establishing or specifying the online status of a user forming part of an instant messaging system. The IM system (11) includes a plurality of clients having IM client applications of the same or different types, including: a PC-based instant messaging client application program (101): a GSM device (102) forming part of a GSM network: an internet browser-based client application (103); and an email-based client application (104). An IM system server (1) including an IM server (111) and a user information database (112) is selectively connected to each of the clients via the internet (113). The user database (112) stores an activity status for each user of the IM system (11) in relation to its activity with the IM server system (13). The activity status includes: i) a time variable signifying the time of or time since the last prescribed involvement of the user with the IM server: and ii) a probability variable signifying the likelihood of the user still being online. Monitoring means is provided to continuously monitor user activity with said IM server and to continuously update the activity status of users stored on the database (112) involved with the activity. Checking means is also provided to check the activity status of each user stored on the database (112) and adjust said probability variable in a prescribed manner having regard to the time variable.
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