"Facebook is about to disappear"
No, it has merely (finally) reached the peak of stupidly fast growth and now has to take a reality check. Until something comes along to usurp it, FB will remain a big player.
Also remember this is specific to the UK, who are probably like 3% of FaceBook's user-base.
FB is no longer the cool new thing but it is still the dominant player... look how many decades MS has stayed top and how slowly their position is eroded even with Vista and W8.