Twitter Inc. said that to help users find friends also using the service, it retrieves entire address book from users' smartphones, including names, email addresses and phone numbers, and keeps the data on its servers for 18 months.
After questions about the practice, the company said it plans to update its apps to clarify that user contacts are being stored.
The disclosure from Twitter comes after another online social service, Path, came under fire last week for downloading iPhone users' address books without permission. Path's chief executive, Dave Morin, apologized for the automatic download and said Path would correct it, but also mentioned that such processes were "industry best practice."
As with many online social services, Twitter allows users to look for friends who are also registered users. In the case of Twitter's iPhone app, users see a screen noting that when users tap the "find friends" option, the service will "scan your contacts for people you already know on Twitter." The short description of the feature does not mention that it also downloads every entry in the address book and stores it.
Twitter spokeswoman Carolyn Penner said in an email that the company is planning an update to the language they use in the mobile app.
"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," she wrote.
Explore further: Twitter wants users to update their personal info