I think you're missing a step. The surpluses are invested in Treasury securities, which is where the "IOUs" tripe comes from. Figures for the national debt invariably _include_ these securities, which is worthwhile to point out lest people think the liability is somehow hidden with shell games.

I think the latest figures for the national debt are just shy of $14 trillion, of which about $3 trillion is owed to the Social Security trust funds.