The airlines hope the messages will help them fill planes that might otherwise fly with empty seats.
Last month, Jet Blue sent out a tweet offering a last minute $9 dollar one way fare from JFK to Nantucket.
Consumers have to sign up to receive twitter updates from United and Jet Blue.
United has close to 21,000 Twitter followers.
Jet Blue's Twitter account has 988,000 followers. You have to be willing to act fast.
The last minute fares are typically only available for a couple of hours.
