Mark wouldn't be such a millionaire if Facebook would have been invented 20 years before he was even born. :))
2. The sublime, but mighty Twitter
I'm thinking that people would have loved Twitter back in the day. How did Elvis put it? ... A yes: "A little less conversation, a little more action!" ;)
3. Youtube, the champion
40 years after the invention of television, people would have been amazed by Youtube. And since everybody admired the people in the small box of moving images, just picture being able to film yourself and take a shot at becoming a star. Not to mention the promise that comes with the ad: "Your films will last forever!" But might I just make a small correction (since everybody knows how Youtube works): Your most embarrassing moments will last forever!
4. Skype, the fabulous family reuniter
What better way to have "vigorous" family bonds if you are miles away from each other?!