I absolutely agree. There will be a bigger better thing than Facebook at some point. There is always something bigger and better. Facebook will represent a major plateau in social technology, no doubt about it. Facebook changed the social landscape of over 500 million people and counting.
Remember however that relevance is increasingly driven by the user's want of instant, here and now, access to information. Location sharing, augmented reality,RFID, Qr codes, and mobile social games are all very new and exciting. Add to that the ultracool mobile hardware out there from Androids to iPhones and the tee tablet market and I see unlimited possibilities here! It's exciting! There is so much possibility when you look at all of those variables, I can't believe that there wont be newer and shinier platform that catches us by storm! 5 years from today I'm sure we'll know the Facebook killers name...we may already know it.