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 convent…
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.