but there wasn't time to get it finished. We should be including it in the not-too-distant future. This is where I hope to end up with the sharing options at the bottom of OPs:
The social media stuff should be obvious, but if you click the "Email" button you'd get something like this:
On this page you would be able to enter your own set of email addresses, which would be saved as your own personal mailing list. I've used demo email addresses above as part of the prototype, but obviously each individual DUer would be able to enter addresses of their choice.
Hitting the "Create email" button would then open a new email using your own default email program, like so:
Your registered DU email address would be prefilled as the sender and also the recipient (essentially you're sending the email to yourself), and then email addresses you selected from your mailing list would be added as BCC recipients -- this is for privacy, so they won't see the other addresses upon receipt of the email. In the body would be some brief information about the thread -- the title etc. -- plus a link.
Then you can either send it off as is, or you can add your own comments to the email (or change the subject, or add recipients, or change the sender, or what have you).
So what you're seeing above are screenshots of a prototype which has yet to be programmed, but it's something I hope to include in DU4 pretty soon.