What I like about the megaphone is that looks similar to what we are doing, shouting to whoever wants to hear.

When you think about sharing a milkshake, you have two people sharing an experience; when you think about the open hand, looks like you are expecting someone to show up to hand over something to him or maybe begging.

My feeling when I click share or retweet is like I am shooting to stars and the odds that my bullet get to a black hole are huge.

I think the modern icons are striving for a more abstract and minimalistic look. This seems to be the case for all 3 mobile OS covered. This is mostly about fashion, I guess, but an abstract icon does look less out of place in more situation. I can well imagine being slightly miffed about having to put a megaphone in my app to fit platform conventions, although I would gladly take it over that god-awful present icon WPOS7 apparently had.

That's an interesting idea. I vaguely remember coming across such symbol somewhere to convey the idea of sharing but I cannot remember where.

I can definitely see that working. As long as you differentiate it enough from a volume icon.

Off the top of my head, UserVoice and TF2 both use a bullhorn for "Give Feedback".

Heck yeah, good call