What does Ev Williams, co-founder of Twitter, think of App.net, the ad-free social network that costs $50 to join? Not very much. He hasn’t tried it, he said at a panel discussion hosted by Branch, a portfolio company of his investment vehicle, Obvious Corp., last night. Follow up: Does he think advertisers have too much power over platforms? Nope!

The question came in context of Twitter’s recent crackdown on certain developers who’ve built various apps and tools on Twitter’s platform. Those playing in the “quadrant of death,” meaning they pull Tweets into their own service, have been kicked off. The company uses the explanation that it wants users to have a unified Twitter experience across platforms, but many have accused it of placing advertisers’s needs above a good user experience. The company is aggressively expanding its advertising program as it strives to live up to an $8 billion valuation.

“People misunderstand the motives” behind Twitter decisions to remove developers’ access to its API, said Williams, who left in 2011 but is still a board member at the company. The assumptions people make about a platform the minute it has advertisers are ridiculous, he said. “(Twitter is) doing something much more nuanced than that.”

In the long term, Williams “is not bullish” on advertising as the best way for startups to monetize. But for now, there is a viable business around it. “The reason we knew Twitter would be a good business from day one was because there were commercial entities on the platform,” he said.

Sharing is as strong a signal as anything when it comes to ads online. On Twitter, sharing is one of the most compelling things about its ad units. No one knows why thousands of people will Retweet a direct endorsement of a certain brand, but it’s something brands are willing pay for. Clickthroughs on Twitter ads have historically been much stronger than Facebook, from 2010, to this year even as Facebook stresses mobile.

The company’s self- serve ads business alone is expected to be worth $400 million this year. The platform’s users are known to be there for their interests, not for actual social networking with friends, which makes the platform a more compelling play for brands. (Interest graph trumps social graph.) Developers be damned, Twitter is determined to become a viable business.

[Image via Jon Steinberg]