Min Ming Lo analyzes the various designs of the share icon currently in use. He concludes:
The best icon is not the one that is the simplest, nor the one that makes the most sense. Instead, the best icon is one with which most users are already familiar. An effective icon is one that requires minimum effort for the user to translate that symbol to an action.
I think part of the problem with coming up with a good share icon is that the concept of sharing physical items doesn’t directly map onto the concept of sharing digital resources, which oftentimes simply involves posting a link on a public feed. The latter is closer to broadcasting a message rather than dividing up a resource or experiencing a thing simultaneously. Icons with more specificity — e.g. a tweet icon, a Facebook like icon — are unambiguous; they relate directly to a service and the kind of interaction that service allows. Trying to cover them all with an umbrella of “sharing” is difficult, if not impossible, to do clearly.
The problem doesn’t lie in the icon, but in the vagueness of what it represents. For this reason I think Apple’s icon resembling an upload action is probably the best choice because what it represents is not so much sharing but putting the content somewhere else. Whether the full file is uploaded or just the link is given doesn’t really matter, what matters is that the thing in question is transmitted somewhere. It’s also worth considering whether the fight for an icon-only button is worth it, and that it may not be easier and better to just use the words “share” to describe the action. There’s a reason why the stop sign just says “STOP” — you can represent it with something else, but the negative effect on clarity is just not worth it.