Nobody is coming even close to taking them out in their categories and they are collectively dominant in pretty much everything else except hard infrastructure - even then they are gaining ground.
Snapchat could have had a close chance against FB before FB targeted them hard, so you can forget about that. But even then, google capital is an Investor and on top of that is getting $2BN in revenue from snapchat using google cloud!
Name any potential serious startup competitor to any of these. Doesn't exist.
I'm not aware of any major example of this since the introduction of the iPhone.