How do you get answers from people without being annoying?

I'm throwing a party and invited a few people. they are all being quite bad at replying, its a facebook thing so i know that most of them have seen it and just not replied, ive been sending weekly inboxes to people trying to get an answer they keep saying 'maybe' and 'will let you know' but knowing my friends they prob wouldnt let me know and i need to know in advance for food and drink etc. i think i'm being reallly annoying to them and i don't want it to get to that point where they just don't come cos they're annoyed with me! how can i get people to give me a straight yes or no without keep on pestering them? thanks x