Ooh, I like this topic. I think there probably are a few more people now who have MH issues than, say, 40 years ago and social media has a lot to answer for in that respect (I'm thinking in particular of the pro-anorexia sites as a particular example).
As PPs have mentioned, there are a lot of self-diagnoses going on which, whilst I don't think are recorded in any tangible stats, are anecdotal and apparent to anyone on social media (like here) in a way that they would not have been before. I think that this goes hand in hand with the fact that people are also more willing to talk about their feelings now than ever before.
500 years ago there was no provision for MH issues and people were either locked up at home by their parents or turfed out on to the street if the family didn't have the means to support them. Later on, people were sent to institutions and forgotten about in more serious cases, but literature is littered with anecdotal instances of people who, whilst having nothing wrong physically, couldn't get out of bed or wouldn't leave the house. Agoraphobia? Depression? Dementia? Perhaps. We have lots of names for things now that didn't exist in the past, and we don't simply send people off to Bedlam anymore.
We also have a much larger, older population now. MH provision has not really improved and certainly hasn't kept pace with the times. I don't think comparing now with the past provides any meaningful, empirical evidence but it's easy to see why people think that MH issues are on the rise and they may well be right. There are so many facets to the issue, though. Is it that we're more aware? Is it that issues exist now that didn't before? Or do they now just have a name?