In fact, I think twitter does not really reveal those private tweets.

When you create a twitter account, your account is public by default. All the tweets that you have posted before you set your account to be private will be searchable.

This is just my $0.02.

The search probably runs off a separate full-text index in their back-end (an inverted index if you will). When people post public tweets these probably get enqueued to be added to the index, so they will be included in search. They just need to delete all tweets from the index an account once it's made private. Maybe their index is append only, or it's very computationally expensive to remove old records (which is usually true for these data structures), so they've opted not to do this yet.

Thus far they don't delete tweets from the search results even after they have been deleted within twitter itself, so I suspect they are append only as you suggest.

It's frustrating that search includes deleted tweets. It's depressingly often for me that a Twitter search results page has at least one duplicate result because someone posted twice to fix a typo.

I checked again and you are right!

This is also interesting in a way that twitter can actually support individual tweets to be private or public like LJ does for blogs. That would be a useful feature.

That's true, but in addition they are showing tweets that were published post making the account private. I have verified this with a friend who has their updated locked. I am not following them but can still use search to see what they are saying every day.

Yeah, that's what it seems like could be happening. But I don't think thats acceptable at all. Twitter offers the ability to mark yourself as private, but doesn't actually protect everything. The same goes for deleting -- you are given every indication its gone, but it stays in search (where people go to "find" thing, probably the place you want it gone from the most).

Twitter search was originally another company which was purchased by twitter latter on. It recorded and indexed tweets without any magic sauce, but in a way that other developers could as well. Thus, even if search.twitter doesn't feature a given tweet, any arbitrary API user could have it, and there is no way for twitter to remove those. Thus perfection is out of reach, and even if they did delete results from search, it would be more of an act of appeasement than an actual technical solution.

It seems to reveal some, but not others.

A friend of mine at work picked up on Twitter within the past week, and his profile is private. His first tweet shows up, but no others, though I can see a few his friends have retweeted his messages.

I do not have a twitter account, so I was not signed in.

Ooh, that's probably another one of those Ruby problems where Rails isn't scaling correctly....