It’s conceivable that Mark Zuckerberg’s Wednesday blog post “A Privacy-Focused Vision for Social Networking” is a historic document not only for Facebook but for the technology industry, period. Zuckerberg outlines a vision for Facebook where content is encrypted and private. And temporary. And shared predominantly over Messenger and WhatsApp chat sessions rather than via a Facebook wall. In some ways, that sounds like . . . well, the opposite of Facebook as we’ve known it. How often has any huge, profitable enterprise declared it was going to change course so decisively?

Zuckerberg is describing something that’s such a radical departure that there’s no guarantee that Facebook will be able to build it, or that people will want to use it–no matter how fast the company moves or how many things it’s willing to break. Reengineering something as vast and complex as Facebook is tough even if you’re just messing with a single aspect: The company still hasn’t shipped the “Clear History” account-scrubbing feature that Zuckerberg promised last May.

It’s also reasonable to approach Zuckerberg’s new game plan with skeptcism based simply on his track record. In Facebook’s earliest years, he had an uncanny knack for understanding what people wanted out of a social network. He was also willing to push boundaries in a way that intitally left some people uncomfortable. That combination of prescience and self-confidence helped Facebook become a phenomenon as rivals such as MySpace and Friendster dwindled away.

Over the years, however, Zuckerberg has confidently expressed visions for Facebook that didn’t come to pass and were sometimes even dead on arrival. Herewith, a few examples.

2010: “Email is too slow. Email is too formal”

At a November press event, Zuckerberg unveils Messages, an update to Facebook’s chat features that offers every member a Facebook.com address. Zuckerberg says that Messages, which he says will intermingle aspects of instant messaging and SMS and store all conversations forever, isn’t email. Still, many people wonder if Facebook is trying to build a Gmail killer.

As it turns out, few folks want to route their inboxes into Facebook; the company ditches the idea of Facebook.com addresses less than two and a half years later. To be fair, other aspects of Messages–which is later rebranded as Messenger–do take off. Which is why Zuckerberg’s new prediction that Messenger and its fraternal twin WhatsApp will become the primary means of interfacing with the Facebook network only sounds like a stretch, not an impossibility.

2011: “An order of magnitude more connections”

Zuckerberg uses his F8 conference keynote in September to outline a vision in which data about our lives flows into Facebook, and is stored indefinitely, without our ongoing involvement. “You can just eat a meal,” he says. “You can hike a trail. You can listen to a song.” Certain aspects of this sound creepy even as Zuckerberg is getting giddy about them onstage. Soon, it’s clear that even something as innocuous as auto-posting what you’re listening to on Spotify doesn’t accomplish much except to irritate your Facebook friends.