Twitter has offered two explanations for the change. First, that very few users were choosing to receive these kinds of messages anyway and that it was confusing. Then, this morning, the company put up a blog post saying simply that "there were serious technical reasons why that setting had to go or be entirely rebuilt--it wouldn't have lasted long even if we thought it was the best thing ever." So what's the story? Here's our best guess.
First, it is clear that most users have not chosen to receive public replies sent by their friends to people they themselves aren't following. Non-early-adopters in particular are quite likely to consider these kinds of messages noise. Many of us early adopters believed such messages were a part of the magic of Twitter; it's a great way to discover people your friends find interesting. Information overload can be dealt with by forming groups in a third party twitter client like Tweetdeck or Seesmic Desktop. Getting over our antiquated sense of guilt and obligation concerning reading every message we receive would help too. (We've argued that online noise is good for you.)
The fact is though that most Twitter users now probably only receive replies sent to people they know. That's been the default setting for several months and we assume the system has thus been architected for this use case.
What does that mean? Probably this: when you publish a Tweet that begins with @username, the computer that all your user files lives on in Twitter HQ probably already knows not only who your friends are, but also who their friends are. Everyone who is your friend but is not friends with the person you are replying to won't even have the message sent to them. Making that determination locally and limiting the tweets broadcast to your friends is probably much more efficient for Twitter than sending that message to all your friends.
Even updating the "friends of friends" files for all your friends every time you make a new friend - is more efficient than sending out replies to all your friends who aren't following the recipient.
Sending Tweets from one user to another is traffic-expensive and if most people don't want replies directed at people they don't know, then none of us are going to get that kind of message.
Most people don't want a noisy public conversation? Most people don't want serendipitous introduction to new people? That sounds like most people don't want a key part of what makes Twitter most magical. That magic is expensive and if millions of Oprah followers don't even want it, then a few thousand @marshallk followers don't get to have it either.
Who can blame them? Everyone wants Twitter to scale. Maybe it has to be neutered in order to do so. Architecting the social graph can't be easy. We can be sad about the decisions that the company makes in order to try to do it, but that probably isn't going to change things.
Instead of strolling through the social graph to discover new users, Twitter users will likely be given a new set of recommendation features pointing them to new people to follow. It may be a "people you follow also follow these people" type of feature, hopefully it won't just be more recommendations based on who Twitter's established leaders like. Or perhaps those of us who want all messages published by our friends can pay for a premium account. I'm ready to do that right now.
Thanks to the super-smart Alex Iskold, who we trust a lot and who helped talk us through this technical speculation.
Update: Hours after we put up this post, Twitter has reversed this policy. The story just keeps getting more interesting!
Just as soon as Twitter is back up, let's be friends. You can find ReadWriteWeb on Twitter, as well as the entire RWW Team: Marshall Kirkpatrick, Bernard Lunn, Alex Iskold, Sarah Perez, Frederic Lardinois, Rick Turoczy, Sean Ammirati, Lidija Davis, Jolie Odell and Phil Glockner.