I'm one of those who uses it.
I use mostly ), )), ))) - more then that is very rare exception
first is like slight smile;
second is like - my smile lasted second or so, yhea good;
third - my smile lasted few seconds and probably laugh'ed, like this was really funny
more then that - I was laughing a lot, and it was crazy as sh so funny, can't stop to laugh, I have convulsions from that all and my finger do not belong me at the moment.
It is not wide used convention I guess, used myself as example of my reasoning. So it is kinda that battery charging icon - low charge etc.
if you see so much ))))))) more then once a day - girl, young, is annoyed, not ready to reject you yet. I'm kidding). But more seriously they do use another convention, do not know about them too much, would not take serious their smiles trough))).