I'm one of those who uses it.
I mostly use ), )), ))) - more then that is a very rare exception.
The first is like slight smile;
The second is like my smile lasted a second or so, yeah good;
The third - my smile lasted a few seconds and I probably laughed, like this was really funny
More then that, I laughted a lot, and it was crazy as hell, so funny, I can't stop laughing, I'm having convulsions, etc.
I don't think they're a widely used convention, using myself as an example for my reasoning. So it's kind of a battery charging icon - low charge etc.
If you see a lot of ))))))) more than once a day, it's probably a girl, young, annoyed, not ready to reject you yet. I'm kidding) But when they're more serious, they do use another convention, which I don't know much about. I wouldn't take their smiles seriously, though)))