...how about #3?
"Our own viewer users are a minority. A significant minority -- we're the number three viewer behind, behind [Phoenix and Firestorm]... Phoenix is far and away the number one viewer, although it's quite steadily losing market share these days, has been for some months now. And Firestorm is the newer technology viewer from your project, is the number two, and it's gaining market share... And our viewer is number three behind Firestorm." --Oz Linden, March 2012
Check out this article at New World Notes.
It would be even more interesting to know what the share is like if one doesn't count alts, and even better if one could exclude bots (or do they use some text-only client for efficiency?), but even so I'm surprised.
One of the comments on the article claims that it shows how few new users are coming to Second Life, since you'd think they'd start out with the LL viewer... but I don't know whether that's the case. I would suggest that it may mean that the newcomers who stay are those introduced to SL by friends who can guide them through that important initial interval, and that those friends are more likely to point their charges at the viewer they themselves use.