Karloff said:
Last time it did this, more than 3,000 people participated, and stopped playing shooters for a day.
http://www.escapistmagazine.com/forums/read/7.835580-Steam-Boasts-7-Million-Concurrent-Users-Over-Thanksgiving
http://www.escapistmagazine.com/news/view/130219-Microsoft-Says-Youve-Killed-3-Billion-Zombies-So-Far-Go-Kill-More
1.4 million playing Modern Warfare 3 simultaneously. Over 3 billion kills by over a million people in under two weeks. 3000 taking one day off? Yeah, that's totally going to be noticeable. An "event" that has less effect than random variation is no event at all, regardless of whether you agree with its aims or not.
And of course, that's assuming those 3000 are even telling the truth. Players of shooters don't exactly have a history of keeping their word [http://www.destructoid.com/this-is-what-the-modern-warfare-2-boycotters-are-playing-154944.phtml] on these matters, and a Facebook group certainly has no way of monitoring them.