Rate of antidepressant use isn't a good proxy for mental illnesses.
It could be that the US is still under diagnosed compared to nations that have national healthcare systems as part of their tax structure.
It could also be that even with the above the US receives more diagnosis for anti-depressants due to quality of life and interactions between the individual and the culture of a given area.
It is my hypothesis (but I've no idea what terms to use for the search) that the culture in the US is less supportive and less about inclusion and empowerment than it has been in the past or possibly than other cultures. However researching that is much more something that those in social sciences should be performing as part of their education and society's investment in to scientific experiments for the public good.
"About 11% of Americans aged 12 or older take antidepressants" (2011)
"the rate of antidepressant use in the U.S. has increased nearly 400% since 1988."
Many people that are on anti-depressants won't tell you about it.
edit: Really? Downvote me just because I provided a source to the claim that more people are on antidepressants today?