Mary, what I meant by saying some song circles are mainly about "socializing" and learning new songs is that in some song circles (e.g., a couple I know of in Portland and one in Seattle), the social experience of singing together seems to be much more the goal than the aspiration to make exquisite or professional-sounding music.   In such groups, it is not unusual for the person whose turn it is to launch into a song a cappella without really knowing the tune, with no sense of meter or phrasing, etc., and with the resulting "sing-along" being at times pretty chaotic or comical.   Rather than ask if someone else feels comfortable leading it, perhaps with a guitar to help keep people on pitch, someone will just plow into a song (a well-known one or one that's "in the book"), with some people doing one arrangement while others are doing another.   Suggestions made to improve the quality of the group singing (e.g., "Please watch the lips of the person leading the song so you'll know when to come in on the next line") are usually regarded as impertinences which spoil the camaraderie.   
What I meant about a goal being to learn new songs is that in some groups people enjoy hearing a new song even when the one presenting it has to read it from notes or a book and maybe can't even stay on key.    I have sometimes appreciated someone singing a song from RUS or song sheets, even if they sang it badly, because it allowed me to hear what tune goes with the lyrics or because it was just a really nice song, which I might add to my repertoire.
Unless it's a "songwriter workshop," song circles are seldom solely about hearing new songs, but some workshops at music camps do seem to be more about sharing songs on a theme -- for the sake of the songs and the theme -- than making beautiful music then and there.   
Books and song sheets tend to be used in circles like this, either because the one leading/presenting the song doesn't know it well (and, for that group's purpose, doesn't need to) or for the sake of the group singing along.

I was at a song circle last night where the consensus seemed to be that everyone should sing along on everything if they either knew the song (or chorus) or could find it in their book or notes.   I don't think anyone did any solos except for verses, etc., that they did not know, and I don't think solo "performances" would have been all that welcome.   The group seemed to construe it as a "sing-along."