1. A system for establishing or specifying the online status of a user forming part of an instant messaging system comprising a plurality of clients having IM client applications of the same or different types and an IM server selectively connected to each of said clients via a computer network, the system comprising: a user database for storing an activity status for each user of the IM system with respect to the IM server, said activity status including: (i) a time variable signifying the time of or time since the last prescribed involvement of the user with said IM server; and (ii) a probability variable signifying the likelihood of the user still being online; monitoring means to continuously monitor user activity with said IM server and to continuously update the activity status of users stored on said database involved with said activity; and checking means to check the activity status of each user stored on said database and adjust said probability variable in a prescribed manner having regard to said time variable.