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.