define facebook friends?

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I just started on Facebook but I've already gotten friend requests from people I've never spoken to and only recognize faintly from school. I know that Facebook friends are way less tight than real friends so can someone tell me if this is usual Facebook behavior or just a few exceptions that want to extend their friends lists? Also- in general do you suggest friend requesting some acquaintances or just keeping it to your friends (the way I have it now) Thank you for putting up with my lack of fb culture