I actually don't think there is any widely-known e-commerce software today that really centers on such Social capabilities (as opposed to implementing a few social features in plugins or extensions, like Facebook Like, Facebook stores, share buttons, etc.).

There's probably a huge market opportunity there, though. Maybe through co-browsing, maybe through something more akin to a social network for the merchants' customers.

Maybe something like a Quora for merchants?

Which shopping cart has true social capability?