If there is any of your facebook friends playing, they Will be and to your friend list automaticly.

Other then that any manager playing on your server, can send you a invitation, the good things is that you can send and receive gifts from them, can play friendlies, good option to gain boosters and money.

If they are of your level, you can end in the same LEAGUE with them, meaning it Will be more dificult to win it, since all the teams Will be active, but at the same time more interesting to play, specialy If the quality is similar.