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Say I wanna talk to a friend, and I wanna be aware when he or she comes on, so the moment the user comes on, a notification or something is informing the person who has set it.
this seems like it would be very easily abused to harass people, unless you required them to accept it for these notifications to be sent.
In that case, that sounds more like a friends list, which is currently planned (but blocked by the current client preact rewrite).