Google, similarly, will be killed not by a competitor rising out of nowhere, but by falling into irrelevance.
Be careful. What's the quote? "It's difficult to make predictions, especially about the future." People said the same of Yahoo in the late 1990's. It had many competitors but wasn't really challenged for the top spot. Google came out of nowhere and relegated its dominance of search.
"Facebook, on the other hand, doesn't have any such flaws." ... "Facebook is as unkillable as Google." ... "You won 't kill Facebook. No one will."
Facebook and Google usurped their predecessors from out of nowhere. They both wouldn't exist if their founders said to themselves, "I'm not going to try to overtake Yahoo/MySpace -- they're unkillable." Your flimsy advice is now recorded to be ridiculed later because you seem not to understand the very nature of technology.
The next leaders get this sort of insipid advice all the time but, luckily for all of us, they aren't listening.