If they make their goal, everyone that logs into the game during the month of July will receive something special. From the page:

Help us reach 1 million Likes on Facebook and earn some juicy in-game rewards for yourself and your fellow players!

If we reach our goal of 1,000,000 Likes on Facebook before the end of July, everybody who logs into Guild Wars 2 – veterans and new players alike – will get these in-game rewards:
A special achievement
A Communal Boost Bonfire that provides a boost to karma gain, experience gain, or Magic Find for you and your friends
A consumable boost for 24 hours of 200% Magic Find and coins from kills
3 consumable Fireworks to celebrate the moment in style.Tell your friends and Like us on Facebook!
Facebook is one of the strongest advertising outlets in the world at the moment, there's nothing to it.The pkpkg has its own 10000 likes and many players like buying wow gold on http://www.pkpkg.com from facebook. And if you're one of the people who liked their page even before the game even came out, you can just sit back and enjoy the highly possible rewards. Less than 90k likes to go before the end of the month, I'd wager the goal will be reached before the end of the week