Just a quick note to draw your attention to this plugin for Elgg.
What is this?
Like with Known, this plugin provides a handy bookmarklet that will allow you to add friends from other platforms (even non-Elgg ones) simply, providing that the platform marks out their users using Microformats (for this reason, I suggest you also take a look at my Elgg microformats plugin).
Once found, a remote user can be added as a friend.
At the moment the plugin doesn’t do much else, but once you’ve got a user stub to hook things on to, other plugins could provide other features – browse their feeds, for example, or provide PGP login support (which I’ll build if I get time) that’ll allow for cross platform private posts.