I first came to SL in 2006 and back then everywhere was full, I drifted away from SL for a number of years coming back to it again this year, and the difference is huge , back when I used to come on ,SL was full, in fact it was too full to be honest as lag was a huge problem at popular places and back then internet speeds were much slower, so if you went to a popular place it was probably so laggy to be no go anyway, but most Sims I went into would have a decent number of people, it was rare to teleport to one to find it empty, and the shopping area`s were almost always busy.
Since coming back i`ve noticed SL is in many places empty, there are loads of Sims which seem devoid of anyone, and even shops which I used to visit which were packed are now in the main pretty empty.
I think the main problem is that the number of people online in SL is much lower these days, when i`m normally online around 50,000, where as back in day it was around 400,000. SL has kept growing land, but with a reducing population it just means you are trying to spread to little butter over to much bread, and that leads to many areas which just don`t have any butter and those places that do get some just get a very thin coat.
I`m not sure there is a solution to the problem, the whole SL model was based upon selling land and growing the population and in the early days it worked fine because people were coming to SL faster than it could grow, but now the reverse is true, more people probably leave than come in or come back, so you have a shrinking population but the SL model still has to grow land, and that in effect counters attracting new blood back into SL as when a newbie arrives and finds so many places deserted they probably turn from a newbie to a never againer.