Given which already enables users to create and publish content, is there a service or tool which can add the standard social networking suite of components?

Specifically, I'm looking quickly add functionality which allows users to friend each other, vote on/like/rank content on the site, send each other links to parts they find interesting, chat and send offline message each other.

There's no specific limitation on the technology used for these components - as long as its been proven to work, and scales without issue. I'd slightly prefer a solution which is offered as a service rather than one that I have to install.

Edit Some additional commenter requested clarifications - there are no restrictions that the site imposes on user identification or authentication. Feel free to assume that portion of the work is not relevant to the answers.