It seems like "just pick something arbitrary and everyone agree on it" is a better solution than "find a small simple icon that effectively evokes 'share' to a brand new user." In other words, this is confusing not because the icons don't represent sharing well enough, but because they're different on every platform. There are lots of things on computers that aren't immediately obvious, but once you learn the convention, you don't forget it. I don't care which one we use, but we need to be consistent. (Ok, not the milkshake.)
There is no doubt in my mind, that the milkshake is the best possible icon for sharing. The only thing that could conceivably be better would be a Lady-and-the-Tramp-style single-strand spaghetti dinner, but that might be hard to draw.
I grew up drinking milkshakes, but I didn't know that sharing one is a thing in America! So, add another two billion people to that number (highly scientific estimate).
The whole soda fountain phenomenon was before our time, so I'm not sure that milkshakes were available at most soda fountains, but it's easy to imagine the tableau relocated to the ice cream shop. The "milkshake icon" seems more evocative than the "soda icon".
But sharing a milkshake isn't like sharing a piece of music. If I saw the icon below an article, I'd think, "read this article with another person". (Ok, maybe I would think 'share', but that doesn't make it a good symbol.)
There is some irony to the fact that we cannot 'share' what the 'share icon' is supposed to look like.
Unless some decree comes down from The President or the security council of the UN then we will probably have Apple/Microsoft/Android variants for a long time to come. There is no incentive for social networks to have a standard icon because they want sharing to be their icon/logo. Maybe it might be a better convention to perpetuate the share button as being uniquely un-shareable just for meme value.
