Perhaps you caught Marco's interview on Bitsplitting: http://bitsplitting.org/podcast/episode-8-marco-arment/
He makes a good point about building on other people's platforms being risky, as they don't last forever.
What I question though is the underestimation of Facebook, and whether it really is "just another platform" or whether it has reached a point where it's really the platform for all the other platforms.
Horses disrupt walking. Cars disrupt horses. Ships, planes and flying cars disrupts cars.
But if Facebook succeeds in becoming the roads, the airports, the seaports, and the GPS guidance systems for the flying cars... they'll be a little harder to disrupt.
My personal opinion is that Facebook has already passed the tipping point that makes it too hard to defeat. Instead, successful ventures in the future will leverage Facebook's platform rather than compete against it.
But I guess I ultimately agree with Marco that nothing is forever. Maybe not even the internet.