I was going to check availability at some other hotels and when I went to the VISA site, here is the announcement that greeted me:
Due to the unexpected popularity of the Hilton free night offer, hotel inventory subject to this offer booked up much sooner than anticipated and is no longer available. We are sorry for any inconvenience this may have caused. Please note that the $75 USD gift card promotion is still available at Participating Hilton Hotels, see the offer page for more information.