Like I said: risk and reward. From the article it's clear that Twitter was a big benefit to his company. It became the #2 referrer after Google. Given the network effect inherent in Twitter, I bet it would become #1 over time.
That's why he's so upset that Twitter cut him off.
So, yes, building on Twitter is riskier. Building on Facebook is riskier. But as distribution channels they have huge upside.
If the risk is too high don't build and wait for the channels to become more mature.
Building on top of a business is sensible when you are depending on something that's been there for a while, know it's dependable and have a reputation. This twitter does not have.
If the same service as Twitter was provided by Google, Amazon or even Microsoft, you know it's backed by something solid.