I know a few MSWs who don't make $40,000 a year. I know one (in the state of Kansas) who makes around $25,000 per year ($12.00 per hour with frequent overtime).
Since the work that social workers actually do varies so much, social work salaries in turn vary a lot, as well. For someone with her LCSW working in a successful and well-established private practice, you could probably see $70,000 per year (or more), but that's much rarer than a lot of people seem to think/hope.