There'll always be overlap when a competing platform comes into the picture - so any 'death' will necessarily be drawn out.
I like your wave analogy, because it highlights the way that these service-orientated companies are tied to the current socio-economic landscape. This helps to show how competition between companies isn't the most important factor. A change in market conditions can prove just as disastrous (as a good competitor) for a company like Facebook, and this kind of change is probably more likely to signal its demise, because a market-leader's success tends to increase exponentially when conditions stay the same.
I think the most interesting question is; what change would be necessary for people to migrate from Facebook to another platform?
It will happen - but when and how?