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 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.