SAN FRANCISCO (MarketWatch) -- Shares of Freddie Mac(FMCC) and Fannie Mae(FNMA) popped on Monday after Bill Ackman of Pershing Square Management characterized the two companies as essential to the U.S. mortage market. They are in a very simple, low-risk business and they are the only ones that provide that service, Ackman told the Ira Sohn conference. The financial crisis was the obvious exception, but Freddie and Fannie have improved since the crisis and are now guaranteeing safer loans, he said. Freddie Mac(FMCC) shares rose 2.5% and Fannie Mae(FNMA) shares climbed 3% in response to his comments. Both stocks trade in the over-the-counter market after their restructuring following their bailout by the U.S. government