An icon that would work well cross-platform as a standard "share this thing" icon. Could be used to launch ShareKit, or other a "Share To Which Network?" dialog. None of the current icons work too well for a "Share" icon. "edit" is about the closest one.

These are contentious: https://en.wikipedia.org/wiki/Share_icon
(I personally like the Apple box-with-arrow-jumping-out icon)