Users can also share content with the 'everyone' which makes it for public consumption. Users can gain reputation in topical areas. Users can search for public content by topical areas.
See this:
http://nexul.com/social/stream/popular/topic/nexul/You'll see a top contributors on the right, and tags/topics with the most discussion below it.
A logo should try to equally express that openness and the ability for privacy at the same time. The "globules" in #1 do that to an extent, as do also the inner facing communication bubbles in #4.