I don't quite understand why people don't respond.  I understand you're busy.  I understand you weren't sure what your answer would be.  I understand that certain plans might fall through and you were waiting to hear from someone.  But tell me these things!  If it's a yes, it's a yes.  If it's a no, it's a no.  The answer itself doesn't matter; it's the fact that an answer exists.  You won't be raining on my parade if you say no.  Often times I invite other people to do things in an attempt to be social.  If you don't want to go or can't go, I don't really care.  In fact, I might rather do it without you.  But the bottom line is this:  Respond!  If I leave you a voicemail, respond!  It doesn't need to be the same method of communication.  Call, email, text message, whatever.  And it doesn't even need to be an answer.  Give me a "maybe".  I just like to know that you got my message and that it didn't get lost in the nothingness of internetland.