There is a difference between those early dependence and Twitter. Twitter is still young, it's uncertain, and worst it's future is really unpredictable because it doesn't even have a business model to generate revenue yet.
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.
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.
This is just the 21st century equivalent.
Likewise, if you're building a destination website you're dependent on Google, email spam lists (don't get blacklisted!), browser vendors (MSFT and ESPN once conspired to make espn.com for IE-only), and even ISPs (see: Net Neutrality).
The risks are higher on Facebook and Twitter, but the upside is also higher. For example, I launched a Facebook application late last week and it already has 500k users. That kind of distribution is hard to get elsewhere.