My fb feed is full of trite messages saying "if you're suffering, talk to someone, we care, we will look after you blah blah blah." Bullshit. When I was severely depressed I did tell people - my GP, my parents, my sisters, my friends everyone. For the most part I got indifference, annoyance, "what can I do?" "get over it" "you're worrying mum," etc. It was only because DH is a bloody saint who spent hours and hours with me that I didn't kill myself. If he hadn't been there then I wouldn't be still alive.
I'm sure if I had killed myself, my family would have done the whole "I don't know why she did it, we had no idea," fuckwittery when in fact they would be perfectly aware of why I'd done it, they just wouldn't think it was a good reason and they would blame me for being cowardly.
Equally everyone else I know who is/was severely depressed is pretty open about it, in fact, I find that depressed people mention their condition quite a lot. The response is generally fear, a sense of not knowing what to say or how to respond, indifference or even disgust.
IME people who are in the depths of depression don't tell others because they have already told them in the past and got nowhere. Implying that if you are in that state all you need to do is "reach out" and someone will be there and everything will get better it totally inaccurate, very few people I know have had that experience. Even the very good kind close friends I had when I had PND last year essentially ignored the illness. They were helpful, very very helpful on a practical level (visiting, sorting out the kids etc) but they never asked me how I was or how the medication was working, I always had to bring it up and then they would just nod and mutter something encouraging. I don't blame them at all for that and I am extremely grateful for the ways in which they did help. It was actually on MN that I got the best support. Here, I met people who had gone through the same thing, who acknowledged my awful thoughts and feelings without trying to change my mind (the whole, "Oh it's not that bad!" that only makes depression sufferers feel they must be mental if no one else can see what they see) and gave me reassurance that yes it was shit but it would pass. They were here whenever I needed them, unlike RL friends who are understandably busy with their own lives.
Depression is a complex illness that requires specialist help. Talking does help, definitely, but a depressed person can't save or cure themselves by just being more open and talking more. Having someone listen is a welcome temporary relief but it doesn't treat the illness any more than talking to a cancer patient would treat their illness.