Obama wants to "manage" the economy and he doesn't even know how investing works. He attacked McCain's support for privatizing social security accounts. Aside from the obvious rebuttal that the privatization plan only gives people the OPTION to invest in the stock market, there is a second response that hits home against Obama's financial scaremongering. As explained at RedState, people do not invest in social security over weeks or months, they invest over decades.
Most people will contribute to social security over a period in the vicinity of 40 years. Even investing in the stock market over a 40 year period including the entirety of the Great Depression would have yielded a positive return of a couple hundred percent. Don't take my word for it. Check out for yourself here.