Twitter and Facebook won't disappear anytime soon by any means. However, both services might be at the peak of their popularity. More and more social media sites are popping up and taking users from both of these services. With that in mind, they're going to be around for years and years to come regardless.
Yes, Facebook and Tweet are here to stay for generations, because they are very highly used. With facebook, many people have uploaded years of family photos and videos. It would be cumbersome for a person to take all of their photos down, and the quality of the photos might not be the same. People use facebook to maintain business relationships, and because it is such a staple, it will remain for a long time.
Although Facebook and Twitter are the social media darlings of the moment, they will eventually go the way of MySpace in favor of the next big thing. In fact, they already are. A recent poll revealed that teens and tweens are leaving Facebook because they see it as a place overrun with "older" people. The downfall of Twitter will likely be laws that re-define what can and cannot legally be Tweeted.
I do not believe Facebook will be with us for many generations. I believe fewer and fewer people are interested in Facebook and I believe fewer and fewer people will use the platform. I think Twitter has taken ahold slowly and is more apt to have a large presence in the future.