Someone sends an instant message "Hi" to you, and you reply, "Hey! How are you?" -- and then... you wait around.. and nothing. Nothing for like forever. Then they go Idle, and leaves you with a big case of WTF?
Classic case of "hit and run".
Also, can be used when you are having a instant messaging conversation, and they leave you hanging unexpectedly...
Last time I talked to her on Facebook Chat, she did a hit and run.