hi dear Sylvia,
you mention an example from Tapped In where participants would signal their level of active presence in a sentence. I believe this habit was inherited from the old MOO text only interface with a strong tradition for acting as-if roleplay, also pointing at a sort of courtesy politeness that was to be expected .I recall one Japanese Tapped In participant who was always entering a room :bowing deeply. This could also be used for a lot of fun - there used to be many shortcuts that you could programme individually, such as standard greetings, or inventing a full costume that would show if people took a closer "look" at you! Such knowledge had to be learnt from others in a social context (or read from the help files on your own).
On your question about just total invisibility I'm not so sure I would like that myself as a facilitator. In an actual case which is not extremely succesful we have very little openly visible participants - but at least they do make their footprints around in the Moodle; in this case there is also a matter of funding to worry about. We do allow guests, but at least they can be viewed in the full activity report. These matters are delicate. And what are our succes criteria - one thing that I know for sure is that it can be harder to document what's going on backstage as activity derived more or less directly inspired from the open and visible conversation, who's calling who on Skype and sending individual emails, Googling and blogging independantly with some more or less indirect connections, etc