First, let me make it absolutely clear that I have nothing against Facebook, and I don't really expect it to be killed any time soon. With its vast membership, it's the ideal place to re-link to old acquaintances. Even if it totally sucked (which it doesn't, though it does have its flaws), people would still stay there because that's where all their old friends are. Also, I have nothing against Mark Zuckerberg. I have him in my circles at Google Plus, just like everybody else does who has a presence there.
The phrase "The Next Facebook Killer" actually implies that Facebook isn't going to be killed. If some "Facebook Killer" were somehow to actually succeed, how could there be a next one?
The phrase was most recently attached to Google Plus, before that service actually launched and when there was speculation about whether it would actually be called Google Plus, or maybe Google.me; but now that plus has actually launched it can no longer claim to be "The Next Facebook Killer". Instead, it has graduated to being "The Current Facebook Killer". This leaves a gap in the market, which needs to be filled quickly. I'm talking about the NEXT next Facebook killer.
So, what should the NEXT next Facebook killer look like? I have a few ideas about that, but I'll stand back for a short while to see what you have to say. For now I'll just say that it should be called Fregobo, because - why not? It's the sort of random collection of syllables that all the most successful sites have nowadays, and it rolls easily off the tongue. So all it needs now is for you to agree with that assessment, and get the domain registered, and power it to the top using the suggestions that are bound to appear here, or with your own suggestions if you prefer. Seriously, what are you waiting for?