Death by a thousand paper cuts may be a fun way to look at how Facebook could eventually bite the dust, but I think it's simpler. Eventually companies are going to figure out that they need to stop turning things inside out and technology will carry human social interaction instead of human social interaction carrying technology.
The distinction can be subtle, but consider a world where you don't think about the company and technology enabling you to share a moment with a loved one. Do you think about which phone network is carrying your phone call (until it drops)? In the future you probably won't think about checking Twitterbook+, it will all just happen.