Leaked documents have shown that that several prominent US web companies collaborate with the US government and provide direct, indiscriminate access to much of the information they store.All of the companies involved have denied being part of the PRISM program, and even knowing anything about it.
But the leaked documents clearly list all companies involved, and even the date they joined. Microsoft was the first company to join, shortly after the program started, followed by Yahoo.
Google, Facebook, PalTalk, YouTube, Skype, AOL, and Apple followed, with Apple joining only last October. Dropbox is said to be coming soon.
One company conspicuously missing from the list is Twitter. Considering that the site is constantly used by activists and alleged "terrorists" alike, and that it was heavily used during the Arab Spring, it should be a good source for the government.
But Twitter has always championed user privacy and rights. It fought the government and it fought the police every step of the way when asked to reveal data.
Twitter does comply with legal requests, obviously, but the PRISM program is voluntary and companies join on their own accord. Though it could be argued that the government would find ways to make them join if they didn't want to.
And, just because Twitter is not on the list, it doesn't mean the NSA isn't able to get whatever it wants from the company. After all, the court orders that mandate these data exchanges come with a gag order that prohibits the company from talking about it.
There's also the fact that much of the data on Twitter is public and the vast majority of tweets are for everyone to see. Followers, the people you are following, most of these things are public by default. Still, Twitter was the most lauded company in a recent report by the EFF.