Yahoo! announced yesterday that a billion user accounts were hacked in 2013 (link). This follows last year's announcement that 500 million user accounts were hacked.
A few websites are saying that Yahoo! users should close down their accounts (link 1, link 2). But, those sites are saying that you should close the barn door after the horse has left. If you had an account with Yahoo!, and your account information was taken then closing your account won't prevent your information from being distributed.
Closing your account however would signal to Yahoo! that you do not trust their security, and there are questions a plenty about Yahoo! data security measures. If you want to close your account, Yahoo! provides a webpage with notices about closing your account, and you can close your account at the webpage "Terminating your Yahoo account".