What I mean is to put a user's online status on their profile. For example, if you were to visit my profile you would see something that says "This user is currently Online" because I'm online. But if I were offline it'd say "User is Offline" and then it would say the last time I was online. If I'd gotten off seven minutes prior it would say "User is Offline, last Online 7 minutes ago." Or something like that. It wouldn't really mean they were on chat, just that they were currently on the site. Of course there could also be a "User is Online but idle" if the user hasn't done anything within a set amount of time.
Of course I don't really know how programming a site works, so I don't know how buggy this could get, but I'm sure that it would be useful!
Ok well I just realized that I put into thought NOTHING when making Meta posts. We have walls.. So yeah. Chances are that most users check their profile wall daily. Sooo... derp! But even so, there are in fact still good ways we could still use the notify button. But now that I've remembered the walls on profiles I can't think of many. But yeah, I think of the notify button as useless now :c i'll just delete that part, but I'll leave this little edit there for explanation.