Our previous share icon was two arrows facing diagonally in opposite directions. The main problem with that was that it was very close to our "embed" icon. We knew we wanted to change it, but we didn't know what icon to use.
We had a bunch of mockups that included some of the icons found in this article.
Ultimately, we ended up deciding on a paper airplane. It definitely is familiar to people in terms of sending email, but we also thought it was a playful and fun way to indicate sharing. Really it was the only icon that we all liked.
It might not be immediately clear at first, but hopefully after using it you get the hang of it.
You can check it out here: https://vimeo.com/76979871
And yes, I am the one in the opening shot of the video who throws the airplane :)