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.
http://www.best-norman-rockwell-art.com/images/1921-Norman-R...
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".
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.