Presently employees and employers each contribute 7.65% of gross wages to FICA. Under the Fair Tax this is covered by the 23% tax on retail sales of new products.
It is true that 15.3% is the total SS/M tax on WAGES -- although the 12.4% SS portion only applies up to the current $90K cutoff. The 2.9% Medicare tax goes on forever.
It is also true that the FairTax will replace this revenue.
It is NOT true that 15.3% of the 23% FairTax goes to SS/M. The FairTax BASE is much larger than the SS WAGE BASE, so it requires a smaller percentage to raise the same revenue.
Oddly enough, the percentages are almost exactly reversed. About 15% goes to general fund and 8% to SS/M. Totaling 23%.