I alluded to this in my liveblog. Dean Baker elaborates:
However, reporters can freely impugn the financial health of a government program like Social Security because a government program cannot sue for libel. That is why Brokaw knew that he could imply that Social Security is going broke, even though it is not true. Social Security cannot sue Brokaw even if he deliberately tells explicit lies about its financial health.
Those who are interesting in learning about the true state of Social Security’s financial health can find out by looking at the non-partisan Congressional Budget Office’s website.