Often used on Fark.com as a ending tag line to a headline that is used as a scare tactic by the media. The over dramatization causes the reader to feel the full effects of the sarcasm and coulis sense of humor the submitter intended for the headline.
In formal usage it can be an exclamatory phrase to express very real danger and incite chaos to the crowds causing mass damage to area's and buildings as well as the people panicking. Please don't yell "Everybody Panic" in the middle of a large crowd as it is an offense that could get you arrested. However funny it might be.
F-15 groundings have caused Alaska to depend on the Canadian Air Force for protection. EVERYBODY PANIC No really: EVERYBODY PANIC! -- Fark