I think part of the problem is the many ways we now receive invites, and the fact that we seem to get invites left right and centre. People seem to have different "standards" for rsvping depending on the nature of the invite.

Eg:
Posted/written invite: I would definitely RSVP and make sure I did it on time or early
Facebook invite: Probably not unless I wanted to go. I get invites to things for people who I barely know or have anything to do with - I wish they would filter their invite lists.
Text invite: I would not deliberately NOT rsvp, but I wouldn't stress too much if I forgot, especially if it was a group text.
Email invite: as per text

These aren't my hard and fast rules, but just trying to show how I approach different invites. Some may consider not rsvping to a FB invite rude, but I think it's rude to send them in the first place if it's the kind of event where you really need rsvps. Does that make sense?
Really? A good friend sent out email invites for her babies 1st birthday party asking for RSVP's. Should I have not RSVP's because she didn't have time to do up a nice printed invite like what I did? Clearly there is some difference between a bulk random invite everyone you know party invitation on facebook and a 1st birthday party or an invite from a friend to anything they are putting on? Another example is a friend is putting on a pre christmas bbq. Yeah bbq's are informal and so may have been the text message but they are still supplying food and they need to know how many people they are cooking for! They are my FRIENDS and I don't want them catering for half the people they invited 'just in case they show up' because they were too rude to rsvp.