Riot Games wants to build a huge following for League of Legends, and its willing to flood the closed beta of the game with Facebook users to make that happen.
Riot Games wants 40,000 folks to follow the League of Legends Facebook group, or become fans, or whatever it is you Facebook people tend to do. And once they reach that goal, all 40,000 fans will receive entry into the closed beta test for the action-RTS. This means that once they reach that goal, if you ask another League of Legends tester for their Facebook and they say they don't have one, there's a good chance they are lying.
To become a fan of League of Legends on Facebook, go to the official page and do that fan thing.
You kids today, I swear.