Fannie Mae's stock price has fallen so dramatically in the last week because of a combination of factors -- shorting by Wall Street hedge funds, a general downturn in the mortgage market, some ill advised comments by a public official, and rampant rumor and speculation cultivated around the firm by the media.
Fannie Mae is adequately capitalized. Defaults comprise <1% of it's total book, even in the current market. It continues to have the backing of the government.
The stock is undervalued. Buy it now, hold, and in a year or two, you will have made an enormous return.