Most businesses can't "die" suddenly. It's very hard to turn a $850,000,000 business into a $35,000,000 in just a couple of years, unless something truly catastrophic or industry-changing occurs. The exception to this rule is social internet companies. Social companies are built in a day and can die in a day - we've seen it happen again and again, and Facebook's scale is no protection. To beat Verizon, you need a net…
In fact, I'd say it's a lot easier to build another Tesla -- who moved into a shuttered NUMI factory -- than another Facebook. It's a lot easier and cheaper to hire machinists, fabricators, welders, millwrights and assembly workers than it is to hire developers. And there's no network effects to selling your product.
Facebook began earning the mindshare of people outside college campuses 7 years ago. Hardly a day. And you say "sical internet companies" as if Facebook can be compared to other social networks. Truth is, facebook has market penetration (in mindshare and daily use) unmatched in internet history outside of Google (and Amazon I suppose).
I think that G+ will be successful. But i think it'll be successful alongside Facebook, and that Facebook will be a strong, profitable, influential company for a long time to come. And that Facebook isn't nearly as easily broken as you claim: In fact, I think there's only one company capable of really threatening it, and that is of course the company that is doing just that with G+.