Facebook was seen as revolutionary when it first appeared on the scene some years ago, but since then the society, especially that of the young, has become even quicker and less patient than ever before. Twitter fills that need with its fast paced means of texting to many people with just a few words.
I do not think that Twitter will end up replacing Facebook as the most popular social media platform. I think that they will both be popular for a while and are on the right track for what people want and like to use but they will not replace one another.
While the user populations of Facebook and Twitter have considerable overlap there is no danger of Facebook being replaced by Twitter as the number on social media platform. Twitter's limit of 140 characters does not allow for any true social interaction among groups of people. It's more suited to mass distribution information and data.
Twitter is a great platform for anyone to use because it is quick and can reach a lot of people. But Facebook has certain features and personal touch that twitter just cannot replace meaning better interaction and relationships for the people on the web. Twitter may be able to come close but overall it would be very difficult to replace Facebook.
Twitter will not end up replacing Facebook as the most popular social media platform because these sites work with different aims. Twitter is about constant updates while Facebook is more conversationally social. If Twitter were likely to replace Facebook, it would have happened before now, rather than co-existing side-by-side with each other.