[lucaspiller]

I don't think you can make something like this fully distributed with current web technologies, but one solution would be to use a centralised domain for identification.

When Person A signs up / logs in their instance includes an iframe at personident.com which sets a cookie saying their instance is persona.com.

When they visit Person B's site the include an iframe for the share endpoint at personident.com. That knows about persona.com, so displays a button to share to their instance.

Iframes may sound nasty but this is how all the social sharing buttons work at the moment. I think the code on personident.com could just be JavaScript, so hosting would be easy.