I also hate how people don't RSVP or just say "maybe". Look in your calendar and make a decision. You either can make it or not. People only say maybe just in case something better comes up. Non-committers!
OMG - How annoying!!!!!!! I don't know about others, but to me this not only shows a lack of respect but also (For me) a lack of how much a friendship is valued. Especially if they do it on a regular occasion.

One of the ppl not coming tomorrow is a friend who had a girls night in not that long ago...now while i was still REALLY sick with m/s and had been vomiting pretty much all day i still managed to go even if it was only for 2 hours instead of the whole night! I at least made an appearence! I don't care if people don't stay...at least make an appearence.