I sent out invites on FB to a birthday/housewarming party that I'm hosting at my house. I sent the invitation out 2 days ago, and Facebook shows when someone has seen the invitation. Everyone has seen it, but only 18 people out of 40 have RSVP'd. I realize it's only been two days, but I've been through this before, and usually people who don't RSVP right away either wait until right before the party or they never respond. Does this frustrate anyone else? Why do people wait so long to say something? Is there a way to gently encourage RSVPs, since I'm trying to plan how much food/drinks we need to serve?