As someone else who has battled depression most of my life I have to make it clear that depression is a broad diagnosis. Robert Sapolsky discusses this in depth and why this makes it such a difficult disease to treat[0]. Hotlines will help some people but not for others. Diet and exercise helps some but not others. Medication helps some but not others. There is no singular solution to depression and framing it that way is one of the worst things we can do. We shouldn't be teaching people that depression looks a certain way because it doesn't. It's why people were surprised by Robin Williams and Anthony Bourdain, despite all the signs being there.
Also, we should also be clear that you are talking about toxic behavior. You isolating yourself when depressed probably isn't the best move.
But I do agree with you about how people react on social media. But I think this is because of the issues above. We discuss depression like it is sadness. So the average person feels empathy at their neighbor struggling but don't know how to help. But I'm not convinced this is criticism for the hotlines, but this should be criticism at how we talk about depression.
So let me talk about depression. For me it can be like a filter is over the world. Everything is just more dull (Anhedonia). It can be difficult to start doing anything, including getting out of bed or things I like (Psychomotor retardation). It's not sadness. It can be a loss of appetite. It can be getting angry and frustrated over things I know don't matter. My friend don't notice unless I tell them. I tend to isolate myself too, but this isn't helpful to my mental health.
If your friends suffer from depression be there. Listen to them. Maybe don't let them isolate themselves if this isn't helpful to them. But it's hard to give good advice because depression is personal and what helps a specific person is also frequently personal. Still, I think hotlines can help. They don't have to help everyone, just some.
[0] https://www.robertsapolskyrocks.com/depression.html