mailto tag will open a new email in their email client with the email address prepopulated, so may not be a great idea, but it'll open the email. It won't forward though, so you can use the Forward to Friend Function if you wish.
As for sharing, it depends. If the shares are ona landing page, then you need to either create custom icons, or use Marketo's icon system. There is documentation on Marketo's system.
For emails, you will need custom share icons. The best thing to do is search for "email sharing icons and links" and then construct the URLs so they pull in the right social handle and populate the link and text the way YOU want it. The lead will still be able to edit it, so keep in mind it should flow naturally.