I was mostly replying to the bit about the general population. In the context of a follow-up question, you might get some quite different results.
Actually, I'm not at all sure if you'd even get a higher percentage of yes respondents than you would in the general population; there's a lot of things I get the sense that a self-diagnosis could be pointing to, most of them likely anticorrelated with seeking formal diagnosis. Charitably, it might indicate an attempt to find out what's going on with one's head in an absence of resources or motivation, or in the presence of social or communication issues or other life circumstances that make one less likely to immediately seek help. Less charitably, it might indicate attention-seeking behavior of some sort, or a trivial approach to the whole issue.
I agree that self-diagnosis could be pointing to multiple, different things. Don't know if there'll be much attention-seeking in the current crowd -- "I'm so cool I'm depressed and I'll say I have MDP to make me extra cool" is a kinda early high school thing and most people grow out of it fairly quickly. People who don't grow out of it are, um, easily recognizable.
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