Twitter has admitted it stores user contact details --- including phone numbers and email addresses --- for an 18-month retention period when the service's "Find friends" feature is used.
New and existing Twitter followers alike can upload their contacts list and address books from a variety of platforms to see if any of their contacts match up to a Twitter registered account. Once users access the feature from one of its smartphone applications, it uploads the data and is subsequently stored for 18 months.
The Twitter application for the iPhone, iPad, and Android devices does not ask for permission before it scans your contacts and reveals your contacts' Twitter accounts.
There are two factors to consider here:
Firstly, authorities have expressed interest on a number of occasions for Twitter data. Birgitta Jónsdóttir, a member of Iceland’s parliament, had her Twitter account subpoenaed by U.S. authorities along with 600,000 other @wikileaks followers. The gagging order injunction on the case was fought and removed with the help of Twitter, bringing the subpoena to light.
The case of the "very British affair" regarding super-injunctions paved way for a UK court to 'subpoena' the names of individuals who broke the court ordered injunction by tweeting the names of celebrities who blocked the entire UK from discussing a factor in their private lives.
Secondly, the authorities are more interested in who you are connected with, rather than what you actually tweet. One counter-terrorism expert highlighted in 2009 in a BBC Newsarticle that it was "contact not content". It's not to say that you won't be barred from entering the United States if you jokingly say you are going to "destroy America", however.
Tweets do last forever, but it is unclear as to how long deleted tweets are retained for. "Log data", such as users IP address, referring page, pages visited, search terms, and even "other actions" and "interactions with our website" may be recorded and stored.
In reaching out to Twitter, to clarify how long other data, including tweet data is retained for, the company did not respond immediately for comment.
A statement to the LA Times, however, spokesperson Carolyn Penner said the company wanted to "be clear and transparent" in its communications with its users.
"Along those lines, in our next app updates, which are coming soon, we are updating the language associated with Find Friends -- to be more explicit. In place of 'Scan your contacts,' we will use "Upload your contacts" [iPhone] and "Import your contacts" [Android]," she said.