That is what this project of the Annenburg Public Policy School told readers today when it backed up its earlier piece claiming that Social Security does contribute to the budget deficit. If did have access to the Internet, FactCheck.org could have easily discovered references to the "on-budget" budget in any budget document it chose to examine (e.g. here and here). The political figures who FactCheck.org criticized in its initial post were obviously referring to this measure of the budget deficit, as was pointed out in a previous note.
An organization engaged in fact checking statements by public figures should probably invest in an Internet subscription so that it can more accurately do its work.