The Earth has 150, 000, 000, 000, 000 m^2 of land. That is a lot of land.
Let's check out SL. According to this graph from the lovely Second Life Economics site:
From my guesstimations, that's about 800, 000, 000 m^2 of land in Second Life. Maybe more, by now. This is just land owned by residents. It might be even more. But on with some math!
Second Life, if it were a planet, would only have 0.000533 % of the land Earth does. For every square meter of land in Second Life, Earth has about 200,000 square meters.
The City of New York has 1, 214, 400, 000 m^2 of land. Second Life makes up about 65.9% of New York. New York has 8,000,000 concurrency. Second Life on its good days gets 50,000 concurrency. Manhattan has 59, 470, 000 square meters. You could fit thirteen and one half islands of Manhattan into Second Life.
Philadelphia has 369, 400, 000 square meters of land. You could fit two entire Philadelphia's into Second Life, with enough room for another fifth of the city. Both Philadelphia and Manhattan have 1, 500, 000 residents, give or take. Second Life has maybe 1,000,000 active residents, with 40,000 online at any given time.
Second Life has a population density of 0.00125 people per square meter. That's 1,250 people per square kilometer. Compare to Philadelphia's (4,201.8 per square kilometer) and Manhattan's (25,846).
What have I learned from these meaningless numbers? One, that Second Life is large on the scale of cities, but not nations or planets. We have enough to maybe cram together a Dallas or a Phoenix. Second, Second Life, while being large, has the population and density of a small city.
While unimpressive on a national scale, SL would be very noticeable on a city-level.