Great point you make there. I think a lot of it shows how society has changed over the years. Mental illness is no longer taboo.... People are encouraged to talk about it now. Even as few as 20 years ago, depression was swept under the carpet and people couldn't talk about it, so you found fewer people did, so it seems as though it is more common in todays world. I don't think this is the case. I think its just that more people will admit to it.
I do feel (especially in England, I don't know about the rest of the world) that doctors are too quick to diagnose depression now though, and not just a case of the "blues". I think true depression and the "blues" are completely different.