I have a hunch that this is a bit like double-clicking the home button on iOS, in that only the minority of tech-savvy users know about it. Successful apps that involve sharing as a core feature (instagram / facebook etc) never rely on the icon alone to convey sharing. It is always accompanied by text or placed inside a big CTA.
I'm interested to know if anybody has any conclusive evidence about the casual user's understanding of these icons.
In the article "Icon Usability" published on July 27, 2014 Aurora Bedford by Nielsen Norman Group states that “Universal” Icons Are Rare".
In particular:
There are a few icons that enjoy mostly universal recognition from users. The icons for home, print, and the magnifying glass for search are such instances. Outside of these examples, most icons continue to be ambiguous to users due to their association with different meanings across various interfaces. This absence of a standard hurts the adoption of an icon over time, as users cannot rely on it having the same functionality every time it is encountered.
Therefore, a best practice is to always present a text label alongside an icon (even standard ones):
To help overcome the ambiguity that almost all icons face, a text label must be present alongside an icon to clarify its meaning in that particular context. (And even if you’re using a standard icon, it’s often safer to include a label, especially if you slightly altered the icon to match your aesthetic preferences or constraints.)
I don't know if this article can be considered the "conclusive evidence about the casual user's understanding of these icons", but given NNGroup's prestige in UX I would take it into serious consideration.
