Yes, they are taking it seriously.

The Facebook event that has over 1 million people signed up to storm Area 51 at 3AM on September 20th has forced the US Air Force to warn against anyone doing so.

Of course, Area 51 has long been the site of UFO mythology. It certainly didn't help that the government didn't even acknowledge its existence until 2013. The "joke" event on Facebook says "they can't stop all of us" and "Let's see them aliens."

Nellis Air Force Base said in a statement that the Air Force is aware of the Facebook posting and says "any attempt to illegally access the area is highly discouraged."

The Air Force says it does not discuss its security measures and that the test and training range provides "flexible, realistic and multidimensional battlespace" for testing and "advanced training in support of U.S. national interests."

Area 51 has garnered the interest of many a UFO enthusiast, even sprouting alien-themed restaurants, alien-themed motels, alien-themed brothels (yup!), and other alienesque tourist stops near the area.

I seem to think that out of the over 1 million people that are "interested" in the event on Facebook, there'll be at least a few thousand that take it seriously, and will show up. Let's hope, for their sake, that there are no injuries in the process. To themselves, to government officials, armed forces, or aliens.