Using the Internet, it's now amazingly easy to invite people to events and track RSVPs. Sites like Ning and Facebook allow this as a feature; MeetUp and Evite are specifically set up with the purpose of creating real world events. I also belong to a number of sites, such as groopy that are designed for specific groups, in this case cyclists, to meet for sports workouts. I suspect there are similar non-jock sites, but I wouldn't know. And plenty of people just use plain old e-mail. Just today I read an e-invitation from someone saying "Please regard this as a personal invitation to..." Real personal, thanks.
As the invitations have become less personal, so have the responses. If you've been to a groopy ride, a Pulver breakfast, high-tech meetup, or FB-organized party, you will quickly see that the number of RSVPs and the number of attendees have a loose relationship. People RSVP and don't show, or show without RSVP, and the number of attendees is invariably 50-70% of the RSVP-ers. Not having asked for RSVPs for my wedding, I don't know how this compares to real life events.
Someone raised the question of whether the RSVP-to-attendee rate varies per site. That would be an intersting study, but my gut feeling is that the rate is determined by (a) the size and type of the event (b) your relationship to the invitee (c) cultural factors.
Two extreme examles are signing up for a dance party on FB, where you have never meet the inviter, and signing up for a 7 am cycle ride in the desert with people you've ridden with before. In the first case, you have pretty much no obligation to come. In the second case, you would send an SMS to notify if you can't drag yourself out of bed, so your friends don't wait up for you.
In all honesty, virtual invitations are ineffective. If you really want people to come to something, you need to call them, ask them face-to-face when you see them, or send something that looks actually personal. After you do that, you also need to send a reminder by SMS on the day of the event. It sounds ridiculous, but I have found that I get reminders for courses that I've signed up and paid money to attend.
At first I wondered about that; If I am paying that much money, you can bet I will show up. On the other hand, your dentist's office also calls to confirm the appointment. Apparently, people who really make a living at this know the honest truth: people forget or skip out even on the important and expensive appointments in their life. If you want them to join your networking event, political rally, or celebration, only the personal touch works.