Official Linden Blog reports that current Second Life land area is approximately 229,376,000 square meters (about 88 square miles or 229 square kilometers). Their front page currently reports 1,950,245 registered residents.
That gives us population density of 8,502 persons per square km. Which puts Second life in top five most populated countries in the world (just above Singapore) and more densely populated when any US state (but still much less dense when city of New York or Manhattan island).
However these figures are misleading. The main difference with Earth population, is that humans stay on earth until we die. On the contrary, the Second Life population stays off grid most of the time. At average, 14,000 to 20,000 residents are usually logged on simultaneously. Taking 20,000 residents as "real" population size, gives us population density of 87 persons per square kilometer. This number is comparable to population density of state of Illinois, and is losing to ten other states, including Ohio, Pennsylvania, and Delaware. On the world map, Spain has similar population density.
Tags: secondlife, population, statistics.