I'd think an arrow to the world would be the right icon, at least if you're doing a "publicize". I think grouping email in with other forms of broadcasting is part of the problem. I'd like to see two icons - one for "broadcast" which would be arrow-to-world (for FB/Twitter/Instagram/etc), and one for "send to a person" which would be an email-like letter icon, (for direct-messaging, emails, SMS, etc). While it may be similar to us developers, to a user the idea of "share" vs "send" is semantically different. "send" operations have recipients. "share" operations do not.