Facebook and Tweet are here to stay for generations. There is not hardly anyone out there that does not have a Facebook account or a Twitter account. I think that ever since these companies have started it has continued to grow and be a everyday activity in most lives. I do not think they are going to go anywhere.
Facebook and Twitter may go quietly into the past but there will always be some sort of social media site around. People thrive on constant contact with others, the Internet has made it possible to share your every thought in a matter of seconds and people love it. I don't see Facebook or Twitter going anywhere but even if they both do, there will always be another website ready to take their place.
I definitely feel Facebook and Twitter are not going anywhere any time soon. Social media has become a very big deal with people and will not die down anytime. That being said, that does not mean a third "big thing" could rise above as well, among the Facebook and Tweets.
Look how quickly MySpace fell off the planet, something will eventually come along that streamlines things even better and knocks off Facebook and Twitter. Nobody will see it coming either, it'll just pop up one day and within a couple of months already be at the top of the mountain.
Facebook , and Twitter have changed the way we relate to each other. They bring an immediacy as well as a distance to our interactions. While Facebook was not the first social media site, it was one of the first to really have a widespread effect. That indicates that there will be other sites after Facebook and Twitter "the next big thing" they may be functionally similar, but they will have evolved and changed.