Have a look at our story from 29-Dec-2006 about Second Life user numbers and a comparison with other Online Games at http://www.duvet-dayz.com/archives/2006/12/29/251/ . The story shows that SL is just 3% of WoW and about 1% of the most successful multiplayer game. Link and charts included.
We compared it to a town stretched out over the whole world. With a population densitiy as high as a national park in Canada no wonder you never meet anybody.