So when I say facebook will fade away then this doesn't imply that it will fade away in someone else's light but it means like a dying sun it will meet its end or maybe it will still have some loyal fans who will keep it alive (just like orkut) but that's not worth it.
Now two very important questions arise why do I think no other social networking site will overshadow it? and Why do I think it will start diminishing itself?
The answer to first question is simple. Till date no other social networking site has been able to even threaten its position. For years now its has maintained its top position and will also maintain it because no one is even competing with it rather other networks has made it even stronger by allowing cross platform sharing and bringing in apps to increase their presence.
The other leading social networks like Linkedin and Twitter are not in direct competition with it and both of them gives a facebook sharing option which in a way promote the giant and make it look even stronger.
Now why do I think it will fade away? If we see the current facebook platform , which though intends to promote the whole life of a person through its timeline but has miserably failed on doing that, why do I say this? because facebook though wants people to look at other's life but its home page only shows what are the recent activities of their friends and pages - which in turn puts pressure on people to update something new and in very small intervals of time to be in the limelight of their friends. This though is good for business as more number of posts are created but in long run it's wearing people out and hence you may notice a trend where people are deactivating their accounts for months just to take a break and be way from all this. There is also very huge number of people who have deleted their accounts or have created a new one after deleting their old one just to be with minimum number of "friends".
One more thing that will definitely cause problem for facebook will be the number of fake users. It gives them as well as analysts a inflated number of users