The US Census released some info about the continuing decline of St Louis, MO's population. Streetsblog blames sprawl, nextSTL likens the population decline to a bomb and worries that all the efforts to rebuild the city have failed, and New Geography mentions that the gentrification efforts haven't paid dividends. This is all potentially distressing for St Louis, but it may not be as alarming as initially feared. What matters is who is moving out, not how many.
Household and family size have been declining in St Louis for quite some time, as is the case many places. But much of the transit-oriented, "urban" lifestyle that the above posts mention is not a major positive influence on family location choices (things like school quality may have a larger influence). About 25% of the 2000 population was under 18, and new denser developments are not necessarily well suited for growing families. We don't know the age breakdown of the 2010 census yet, but it may be that a disproportionate share of the population loss was through youngsters. Potentially enough to off-set any gains of single adults.
We see this is many cities that are realizing revitalized downtowns. The new families moving in are substantially smaller than the families being replaced. New condos and other construction gets filled with more households but fewer people per unit than the city average. For instance, consider a family of four that was two parents and two teenagers in the suburbs in 2000. The kids grow up and move out and now each have an apartment in the city, plus the parents move to the hip area so they don't have to mow the lawn anymore. In 2010 this family would be three households, and they may all have bought new condos in a gentrifying area. Yet if they replaced one five person household the city has a net loss of population even though there are more households and a more active housing market.
I don't know to what extent this is happening in St Louis, and my initial guess is probably now that much, but after the past decade of urbanist development in pockets around the country we ought to be able to estimate some effects from shifting housing types. In the past decade St Louis has invested in transit and supportive developments. These investments will have localized effects, so we shouldn't yet jump to conclusions that these efforts have universally failed. There are likely some areas of St Louis much better off than they were ten years ago even while some are worse off.